Exhaust purification catalyst, exhaust emissin control device for internal combustion engine, and exhaust gas purification filter

ABSTRACT

Provided are an exhaust purification catalyst which purifies an exhaust gas discharged from an internal combustion engine and in which high catalytic activity at a low temperature and high durability at a high temperature are compatible with each other, and an exhaust emission control device for the internal combustion engine in which the exhaust purification catalyst is used. The exhaust purification catalyst is a catalyst in which a noble metal particle is carried on a surface of a silicon carbide particle. The catalyst is a noble-metal-carrying silicon carbide particle ( 1 ) in which a noble metal particle ( 3 ) is carried on a surface of a silicon carbide particle ( 2 ) in a state of being coated with an oxide layer ( 4 ).

TECHNICAL FIELD

The present invention relates to an exhaust purification catalyst thatpurifies an exhaust gas discharged from an internal combustion engine,to an exhaust emission control device for an internal combustion enginein which the exhaust purification catalyst is disposed in an exhaustpassage of the internal combustion engine, more particularly, to anexhaust purification catalyst that efficiently purifies carbon monoxide(CO), hydrocarbon (HC), nitrogen oxide (NO_(x)), particulate matter(PM), and the like which are contained in an exhaust gas discharged fromthe internal combustion engine of an engine and the like, and to anexhaust emission control device for an internal combustion engine inwhich the exhaust purification catalyst is disposed in the exhaustpassage.

In addition, the invention relates to an exhaust gas purification filterthat is very suitable for removing particulate matter from the exhaustgas discharged from an diesel engine of a vehicle, and the like, andmore particularly, to an exhaust gas purification filter which preventsan increase in pressure loss while improving particulate matter trappingproperties, and which lowers the combustion temperature of theparticulate matter during regeneration of the filter, thereby realizingimprovement in durability and continuous regeneration of the filter.

Priority is claimed on Japanese Patent Application No. 2011-185048,filed Aug. 26, 2011, Japanese Patent Application No. 2012-137914, filedJun. 19, 2012, Japanese Patent Application No. 2011-185047, filed Aug.26, 2011, and Japanese Patent Application No. 2012-137913, filed Jun.19, 2012, the contents of which are incorporated herein by reference.

BACKGROUND ART

Various substances such as carbon monoxide (CO), hydrocarbon (HC),nitrogen oxide (NOx), and particulate matter (PM), which are containedin an exhaust gas discharged from an engine (internal combustion engine)of a vehicle and the like, are the cause of air pollution, and havecaused various environmental problems until recently. Therefore, anexhaust emission control device, which uses a noble metal element as acatalyst, is used to purify the substances included in the exhaust gasthat are the cause of air pollution.

In the exhaust emission control devices of the related art, ceramicsformed from a metal oxide such as alumina, ceria, and zirconia, or ametal composite oxide having a perovskite structure expressed by GeneralFormula ABO₃ (provided that, A and B represent metal elements) are usedas a catalyst carrier, and a material in which a noble metal elementhaving a catalyst performance is carried on the catalyst carrier is usedas the catalyst. The substances contained in the exhaust gas are broughtinto contact with the catalyst to perform a decomposition treatment (forexample, PTL 1).

In addition, the particulate matter (PM), which is included in theexhaust gas discharged from a vehicle engine, particularly, a dieselengine, is believed to be a cause of allergic diseases such as asthmaand hay fever.

In general, in a diesel engine for a vehicle, a diesel particulatefilter (DPF) having a sealed-type honeycomb structure formed fromceramics is used as an exhaust gas purification filter that traps theparticulate matter. In the DPF, both ends of a cell (gas flow passage)of the honeycomb structure formed from the ceramics are sealed with acheckered pattern, and when an exhaust gas passes through pores in apartition wall of the cell, the PM is trapped (for example, refer to PTL2 and PTL 3).

However, the PM is always discharged from the engine as the vehicletravels, and thus the PM is deposited in a layer shape in and over thepores of the partition wall of the DPF.

Additionally, when the vehicle travels, a pressure loss of the DPFincreases due to the deposition of the PM, and thus a load affects thetravelling vehicle. Accordingly, it is necessary to periodically removethe PM that is deposited in the DPF by using certain means to regeneratethe DPF. In the related art, in a case where the pressure loss of theDPF increases, a high-temperature exhaust gas is allowed to flow to DPFto combust the PM that is deposited, thereby regenerating the DPF.However, in this regeneration method, the PM that is deposited iscombusted at a high temperature of 600° C. to 700° C., and at an initialstage of the regeneration, the PM is combusted at a further highertemperature. Therefore, the partition wall of the DPF tends to be brokendue to a thermal stress that occurs during the combustion. Accordingly,it is necessary to decrease the thermal stress that occurs duringcombustion of the PM to prevent the partition wall from breaking.

With regard to the DPF, an exhaust gas purification filter, in which afilter formed from a porous substance of silicon carbide is formed onthe partition wall of the DPF to prevent an increase in pressure lossand to decrease the thermal stress while improving the particulatematter trapping properties, is suggested. According to this DPF, abruptcombustion at an initial stage is prevented, and thus a rapidtemperature increases in the DPF is prevented, thereby preventing theDPF from breaking (PTL 4).

However, in this method, the PM, which is trapped by surface layerfiltration using the porous silicon carbide filter, is heated andcombusted in a uniform manner on the porous filter, and the combustiontemperature of the PM can be reduced, but it is impossible to omit aperiodic DPF regeneration treatment process.

Accordingly, to omit the DPF regeneration treatment process, there issuggested a method of continuously regenerating the DPF by lowering thecombustion temperature of the PM that is deposited to combust and removethe PM at an exhaust gas temperature. For example, as a DPF capable ofrealizing a method of efficiently combusting the PM, the following DPFsare suggested. Specifically, a DPF disclosed in PTL 5 has a structure inwhich a rare-earth oxide catalyst is carried on the partition wall ofthe DPF, and a noble metal is carried on a front part of the partitionwall. In addition, a DPF disclosed in PTL 6 has a structure in which arare-earth oxide coat layer is formed on a partition wall of the DPF,and a noble metal is carried on the rare-earth oxide coat layer.

CITATION LIST Patent Literature

[PTL 1] Japanese Laid-open Patent Publication No. 5-4050

[PTL 2] Japanese Laid-open Patent Publication No. 5-23512

[PTL 3] Japanese Laid-open Patent Publication No. 9-77573

[PTL 4] Pamphlet of International Publication No. WO2009/133857

[PTL 5] Japanese Laid-open Patent Publication No. 2007-253144

[PTL 6] Japanese Laid-open Patent Publication No. 2006-272288

SUMMARY OF INVENTION Technical Problem

In general, in a case where the above-described exhaust gas is purifiedusing the catalyst (here, the “catalyst” is assumed to contain a noblemetal element having a catalytic function and a carrier. The sameassumption applies through the document), heat is necessary to allowcatalytic activity of the noble metal element to be exhibited. However,for example, the catalyst is at a low temperature immediately afterengine starting, and thus the purification of the exhaust gas becomesinsufficient. Therefore, there is a demand for stable purification ofthe exhaust gas even when the catalyst is at a low temperature, and thusthere is a demand for a catalyst capable of realizing efficientpurification performance even at a low temperature.

As a method of obtaining efficient purification performance at a lowtemperature, it is considered that an amount of a noble metal elementthat is used as the catalyst is increased to expand a contact area(contact probability) between the exhaust gas and the noble metalelement. However, lots of noble metal elements such as a group 9 elementand a group 10 element, which have catalytic activity for purificationof the exhaust gas, are expensive, and thus there is a problem in thatthe manufacturing cost increases.

In addition, a configuration of using composite particles, in which thenoble metal element is carried on the surface of fine particles (carrierparticles) of γ-alumina and the like with a high specific surface areato increase a surface area of the noble metal element and to increase acatalytic active surface, as a catalyst is also known. In the catalystconfigured described above, crystal transition from γ-alumina toα-alumina occurs at a high temperature of 1000° C. or higher, and thusthere is a problem in that the specific surface significantly decreases.

Further, structural change due to the crystal transition from γ-aluminato α-alumina becomes a cause of promoting sintering (grain growth) ofcatalyst component. In addition, in a case of forming the catalyst in alayer shape on a base material, the structural change in the catalystdue to the crystal transition becomes a cause of generating peeling-offor detachment of a layer that is formed.

As described above, it is difficult for the catalyst using the fineparticles of γ-alumina and the like to maintain catalytic activity at ahigh temperature.

The invention has been made in consideration of the above-describedcircumstances, and a first object thereof is to provide an exhaustpurification catalyst which purifies an exhaust gas discharged from aninternal combustion engine and in which high catalytic activity at a lowtemperature and high durability at a high temperature are compatiblewith each other, and an exhaust emission control device for the internalcombustion engine in which the exhaust purification catalyst is used.

In addition, with regard to the exhaust gas purification filter, in themethod of continuously regenerating the DPF, which is disclosed in PTL 5and PTL 6 of the related art, a temperature of an exhaust gas, which isdischarged from the diesel engine at a typical traveling range, is aslow as 150° C. to 350° C., and thus a high oxidation rate is necessaryat a low exhaust gas temperature range to regenerate the DPF bycombusting the deposited PM in the exhaust gas. Therefore, it isnecessary to increase the oxidation rate of the PM in the DPF, and thusit is necessary to increase the carried amount of the noble metal in theDPF. As a result, there is a problem in that the price of the DPF isincreased.

In addition, when the carried amount of the noble metal in the DPFincreases, there is a new problem in that the sintering (grain growth)of the noble metal particles is promoted, and thus the oxidation rate ata low exhaust gas temperature range decreases, and oxidation andpurification performance decreases.

The invention has been made also in consideration of the above-describedcircumstances, and a second object thereof is to provide an exhaust gaspurification filter which prevents an increase in pressure loss whileimproving particulate matter trapping properties, thereby realizingimprovement in durability, lowering of the combustion temperature ofparticulate matter, and continuous regeneration of a filter.

Solution to Problem

The present inventors have made a thorough investigation on an exhaustemission control device for an internal combustion engine. As a result,they have found that when using noble-metal-carrying silicon carbideparticles in which noble metal particles, the word “particle” may bereferred as “fine particle” hereinafter, having catalytic activity arecarried on the surface of fine silicon carbide particles in a state ofbeing coated with an oxide layer as a catalyst disposed in an exhaustpassage of an internal combustion engine, high catalytic activity at alow temperature and high durability at a high temperature are compatiblewith each other, and thus they have accomplished the invention.

That is, according to an aspect of the invention, there is provided anexhaust purification catalyst in which a noble metal particle is carriedon a surface of a silicon carbide particle. The noble metal particle iscarried in a state of being coated with an oxide layer.

The oxide layer may be formed from one or two kinds selected from agroup consisting of amorphous SiO_(x) (provided that, 0<x≦3) andamorphous SiO_(y)C_(z) (provided that, 0<y≦3 and 0<z≦≦3).

The oxide layer may further contain one or two or more crystallinesubstances selected from a group consisting of SiO₂, SiO, SiOC₃, SiO₂C₂,and SiO₃C.

The average primary particle size of the silicon carbide particle may be0.01 μm to 5 μm.

The average primary particle size of the noble metal particle may be 1nm to 50 nm.

According to another aspect of the invention, there is provided anexhaust emission control device for an internal combustion engine, whichpurifies an exhaust gas discharged from an internal combustion engine byusing a catalyst disposed in an exhaust passage of the internalcombustion engine. At least one catalyst is the exhaust purificationcatalyst of the invention.

In addition, the present inventors have made a thorough investigation ona filter base body which is formed from a porous substance andconstitutes the exhaust gas purification filter. As a result, they havefound that when a porous film, which is formed on a surface of apartition wall that constitutes the filter base body at least on aninflow-side gas flow passage side, is constituted by silicon carbideparticles, and when noble metal particles, which are carried on thesurface of the silicon carbide particles, are carried in a state ofbeing coated with an oxide layer, an increase in pressure loss isprevented while improving particulate matter trapping properties, acombustion temperature of particulate matter can be lowered, and as aresult, it is possible to realize not only an improvement in durabilityof the filter but also continuous regeneration of the filter. From thisfinding, the present inventors have accomplished the invention.

According to still another aspect of the invention, there is provided anexhaust gas purification filter which purifies an exhaust gas byallowing particulate matter contained in the exhaust gas to pass througha filter base body formed from a porous substance to trap theparticulate matter. The filter base body includes a partition wall thatis formed from a porous substance, an inflow-side gas flow passage whichis formed by the partition wall and in which an inflow-side end for anexhaust gas that contains particulate matter is opened, and anoutflow-side gas flow passage which is provided at a position differentfrom that of the inflow-side gas flow passage of the filter base bodyand is formed by the partition wall and in which an outflow-side end forthe exhaust gas is opened. A porous film having a pore size smaller thanthat of the partition wall is formed on a surface of the partition wallat least on an inflow-side gas flow passage side, and the porous filmcontains silicon carbide particles and noble metal fine particles, andthe noble metal particles, which are carried on the surface of thesilicon carbide particles, are carried in a state of being coated withan oxide layer.

The oxide layer may be formed from one or two compounds selected fromthe group consisting of amorphous SiO, (provided that, 0<x≦3) andamorphous SiO_(y)C_(z) (provided that, 0<y≦3 and 0<z≦3).

The oxide layer may further contain one kind or two or more kinds ofcrystalline substances selected from a group consisting of SiO₂, SiO,SiOC₃, SiO₂C₂, and SiO₂C.

The average primary particle size of the silicon carbide particles maybe 0.01 μm to 10 μm.

The average primary particle size of the noble metal particles may be 1nm to 50 nm.

The average pore size of the porous film may be 0.05 μm to 3 μm, and anaverage porosity of the porous film may be 50% to 90%.

Advantageous Effects of Invention

According to the exhaust purification catalyst of the invention, thenoble metal particles are carried on the surface of the silicon carbideparticles, and the noble metal particles are carried in a state of beingcoated with the oxide layer, and thus the noble metal particles areuniformly carried on the surface of the fine silicon carbide particles.In addition, the noble metal particles are carried in a state of beingcoated with the oxide layer, and thus it is possible to secure aneffective catalytic active site. As a result, it is possible to maintainhigh catalytic activity at a low temperature without increasing theamount of the noble metal particles.

In addition, the fine silicon carbide particles themselves havedurability at a high temperature, and the noble metal particles arecarried in a state of being coated with the oxide layer, and thus evenin a high-temperature environment, it is possible to maintain the samehigh catalytic activity as that at low temperatures.

As described above, the high catalytic activity at a low temperature andthe high durability at a high temperature are compatible with eachother. As a result, it is possible to maintain high durability whilemaintaining high catalytic activity from a low-temperature region to ahigh-temperature region.

According to the exhaust emission control device used in an internalcombustion engine of the invention, the exhaust purification catalyst ofthe invention is at least one of the catalysts that is disposed in theexhaust passage of the internal combustion engine, and thus it ispossible to maintain high durability while maintaining high catalyticactivity from a low-temperature region to a high-temperature region. Asa result, it is possible to improve the reliability of exhaustpurification in the internal combustion engine.

In addition, according to the exhaust gas purification filter of theinvention, the filter base body formed from the porous substance isconfigured to include the partition wall that is formed from the poroussubstance, the inflow-side gas flow passage which is formed by thepartition wall and in which the inflow-side end for the exhaust gas thatcontains particulate matter is opened, and the outflow-side gas flowpassage which is provided at a position different from that of theinflow-side gas flow passage of the filter base body and is formed bythe partition wall and in which the outflow-side end for the exhaust gasis opened. In addition, the porous film having a pore size smaller thanthat of the partition wall is formed on the surface of the partitionwall at least on the inflow-side gas flow passage side, the porous filmis configured to contain the silicon carbide particles and the noblemetal fine particles, and the noble metal particles, which are carriedon the surface of the silicon carbide particles, are carried in a stateof being coated with an oxide layer. Accordingly, an increase inpressure loss is prevented while improving PM trapping properties in thefilter base body, and it is possible to lower a combustion temperatureof the particulate matter. As a result, it is possible to improve thedurability of the exhaust gas purification filter, and it is possible toperform continuous regeneration.

According to these effects, it is possible to provide an exhaust gaspurification filter which does not apply an excessive load on an engine,does not deteriorate fuel efficiency, and has excellent characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a noble-metal-carryingsilicon carbide particle that is an exhaust purification catalyst of anembodiment of the invention.

FIG. 2 is a schematic view illustrating a porous film of an embodimentof the invention.

FIG. 3 is a scanning electron microscope image (SEM image) illustratingan example of the porous film of the embodiment of the invention.

FIG. 4 is a schematic view illustrating an exhaust emission controldevice that is used in a test of samples of Examples and ComparativeExamples of the invention.

FIG. 5 is a field-emission transmission electron microscope image(FE-TEM image) of an exhaust purification catalyst of Example 3 of theinvention.

FIG. 6 is an explanatory view illustrating a structure of the exhaustpurification catalyst shown in FIG. 5.

FIG. 7 is a partially broken perspective view illustrating a DPF that isan example of the exhaust gas purification filter of an embodiment ofthe invention.

FIG. 8 is a cross-sectional view illustrating a structure of a partitionwall of the DPF that is an example of the exhaust gas purificationfilter of the embodiment of the invention.

FIG. 9 is a cross-sectional view illustrating an example of a structureof a porous film of the DPF that is an example of the exhaust gaspurification filter of the embodiment of the invention.

FIG. 10 is a scanning electron microscope image (SEM image) of a porousfilm of Example 6 of the invention.

FIG. 11 is a field-emission transmission electron microscope image(FE-TEM image) of the porous film of Example 6 of the invention.

FIG. 12 is an explanatory view illustrating a structure of the porousfilm shown in FIG. 11.

DESCRIPTION OF EMBODIMENTS First Embodiment

A first embodiment for carrying out an exhaust purification catalyst andan exhaust emission control device for an internal combustion engine ofthe invention will be described.

In addition, this embodiment makes a description in detail for easycomprehension of the gist of the invention, and does not limit theinvention unless otherwise stated.

[Exhaust Purification Catalyst]

The exhaust purification catalyst that is an embodiment of the inventionis a catalyst in which noble metal particles are carried on the surfaceof silicon carbide particles. The noble metal particle is carried in astate of being coated with an oxide layer.

The exhaust purification catalyst of this embodiment is suitably used topurify an exhaust gas discharged from an internal combustion engine suchas a gasoline engine and a diesel engine, and may be used in vehiclessuch as a gasoline-powered vehicle and a diesel-powered vehicle.

FIG. 1 is a cross-sectional view illustrating a noble-metal-carryingsilicon carbide particle 1 that is a kind of exhaust purificationcatalyst of this embodiment. In the noble-metal-carrying silicon carbideparticle 1, a noble metal particle 3 is carried on a surface of asilicon carbide particle 2, and the noble metal particle 3 is carried onthe surface of the silicon carbide particle 1 in a state of being coatedwith an oxide layer 4.

One or two or more noble metal particles 3 may be carried on the surfaceof the silicon carbide particle 2.

The average primary particle size of the silicon carbide particle 2 ispreferably 0.01 μm to 5 μm, is more preferably 0.02 μm to 3 μm, and isstill more preferably 0.035 μm to 1 μm.

Here, in a case where the average primary particle size of the siliconcarbide particles is less than 0.01 μm, when the catalyst is used at ahigh temperature, sintering (grain growth) between silicon carbideparticles progresses and a coarse particle size is obtained, and as aresult, a catalytic active site decreases, and thus this case is notpreferable. On the other hand, in a case where the average primaryparticle size exceeds 5 μm, a specific surface area of the siliconcarbide particle 2 decreases, and thus when the noble metal particlesare carried, the distance between the noble metal particles is reduced.As a result, when the catalyst is used at a high temperature, sintering(grain growth) between the noble metal particles progresses, and as aresult, a catalytic activity decreases, and thus this case is notpreferable.

With regard to the silicon carbide particles, examples of a method ofobtaining nanometer-sized particles include a thermal plasma methodusing thermal plasma which has a high temperature and high activity in anon-oxidizing atmosphere and which is easy for introduction to ahigh-speed cooling process. This manufacturing method is useful as amethod of manufacturing silicon carbide nanoparticles which have anaverage primary particle size of approximately 5 nm to 100 nm and haveexcellent crystallinity. When a raw material with high purity isselected, it is possible to obtain silicon carbide nanoparticles inwhich the amount of impurities is very small.

In addition, a silica precursor sintering method may be an exemplaryexample. This method is a method of obtaining silicon carbide particlesby baking a mixture of a material such as an organic silicon compound,silicon sol, and silicic acid hydrogel which contain silicon, a materialsuch as a phenol resin which contains carbon, and a metal compound oflithium and the like which suppresses grain growth of silicon carbide.

In addition, examples of a method of obtaining silicon carbide particleshaving a size distribution from submicron to micron (micrometer) includeindustrial methods such as an Acheson process, a silica reductionmethod, and a silicon carbonization method. In addition, these methodsare already industrially established, and thus a description thereofwill not be repeated here.

The noble metal particles 3, which are carried on the surface of thesilicon carbide particles 2, preferably contain one or two or more kindsof elements selected from a group consisting of platinum (Pt), gold(Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium(Os), and iridium (Ir).

The average primary particle size of the noble metal particles 3 ispreferably 1 nm to 50 nm, is more preferably 1 nm to 30 nm, and is stillmore preferably 1 nm to 10 nm.

Here, the reason as to why the average primary particle size is limitedto 1 nm to 50 nm is as follows. When the average primary particle sizeis less than 1 nm, the particle size is too small and thus surfaceactivity becomes too strong. Therefore, agglomeration tends to occur,and thus this range is not preferable. On the other hand, when theaverage primary particle size exceeds 50 nm, the noble metal particlesare not coated with the oxide layer that is present on a surface of eachof the silicon carbide particles, and thus the noble metal particlesprotrude from the oxide layer toward the outside. As a result, there isa concern that catalytic characteristics deteriorate, and the catalyticactivity at a low temperature decreases, and thus this range is notpreferable.

It is preferable that the oxide layer 4 be an oxide layer which allowsthe noble metal particle 3 to be carried on the surface of the siliconcarbide particles 2 and which is generated on the noble metal particles3 and the silicon carbide particles 2 through oxidation in an oxidizingatmosphere.

The oxide layer 4 has a function of maintaining the noble metalparticles 3 on the surface of the silicon carbide particles 2.Accordingly, the oxide layer 4 suppresses migration of the noble metalparticles 3 under a high-temperature environment, and can prevent adecrease in surface area due to sintering of the noble metal particles3.

It is preferable that the oxide layer 4 be formed from one or two kindssubstances selected from a group consisting of amorphous SiO, (providedthat, 0<x≦3) and amorphous SiO_(y)C_(z) (provided that, 0<y≦3 and0<z≦3). In addition, it is not necessary for amorphous SiO, andamorphous SiO_(y)C_(z) to have a single composition, and at each portionof the oxide layer 4, x, y, and z may vary in the above-described rangein an arbitrary manner. Further, the oxide layer may further contain oneor two or more crystalline substances selected from a group consistingof SiO₂ (silica), SiO, SiOC₃, SiO₂C₂, and SiO₃C.

However, the oxide layer 4 has a too small thickness and is contained ina trace amount, and thus it is difficult to confirm whether or not acrystalline substance is contained in the oxide layer 4. In addition, inthe following description, substances that form the oxide layer 4 may becollectively described as “oxides of silicon carbide.”

In the oxides of silicon carbide, it is preferable to contain a siliconcarboxide that tends to form a bond with each of the noble metalparticles 3, in other words, a compound that contains silicon, carbon,and oxygen in combination with each other. When the silicon carboxideforms a bond with the noble metal particle 3, migration of the noblemetal particle 3 is suppressed, and thus the sintering prevention effectis further improved. In addition, it is known that the melting point ofthe noble metal particle 3 becomes lowered from miniaturization.However, when the silicon carboxide forms a bond with the noble metalparticle 3, melting of the noble metal particle 3 can be prevents.

Examples of the silicon carboxide include crystalline SiOC₂, SiO₂C₂, andSiO₃C in addition to amorphous SiO_(y)C_(z) (provided that, 0<y≦3 and0<z≦3). In addition, the silicon carboxide may contain silicon, carbon,and oxygen in combination with each other, and may include a compositionother than the above-described composition.

In addition, it is known that the oxides of silicon carbide whichconstitute the oxide layer 4 exhibit oxygen release. It is consideredthat the noble metal particle 3 that is carried promotes an increase inan oxygen release amount in the oxides of silicon carbide and loweringof an oxygen release temperature. Accordingly, even in a low temperatureregion in which a reaction rate of a catalytic reaction depends on thenumber of active sites, oxygen released from the oxides of siliconcarbide tends to act as the active site due to an auxiliary operation ofthe noble metal particle. As a result, high combustion catalyticactivity can be obtained, and thus it is considered that PM combustionproperties in a low-temperature region are improved.

[Exhaust Emission Control Device for Internal Combustion Engine]

An exhaust emission control device for an internal combustion engine,which is an embodiment of the invention, is an exhaust emission controldevice which purifies an exhaust gas discharged from an internalcombustion engine by using a catalyst disposed in an exhaust passage ofthe internal combustion engine. At least one catalyst is the exhaustpurification catalyst of this embodiment.

The exhaust emission control device for an internal combustion engine ofthis embodiment is used in vehicles such as a gasoline-powered vehicleand a diesel-powered vehicle. In addition, the exhaust purificationcatalyst may be disposed in the exhaust passage, and a dispositionmethod thereof is not particularly limited. For example, the exhaustpurification catalyst may be disposed in the exhaust passage in a stateof being carried on a catalyst carrying member such as a honeycombmember for exhaust purification, which is used in a vehicle (agasoline-powered vehicle and a diesel-powered vehicle), and a DPF for adiesel-powered vehicle.

The exhaust purification catalyst in this embodiment is preferablyformed and disposed on a base material provided in the exhaust passageas a porous layer that contains a plurality of noble-metal-carryingsilicon carbide particles 1. Here, the porous layer is intended toinclude not only a porous film, which is a continuous film-shapedstructure, but also a porous agglomerate.

The noble-metal-carrying silicon carbide particles 1 may be in a statecapable of exhibiting a function as a catalyst, and thenoble-metal-carrying silicon carbide particles 1 may be in a state ofbeing dispersed in the porous layer. Several noble-metal-carryingsilicon carbide particles 1 may be disposed in the porous layer in anagglomerated state. In addition, several noble-metal-carrying siliconcarbide particles 1 may agglomerate and form a continuous film-shapestructure or a porous agglomerate shape may be formed. Any case iscalled a catalyst layer. In addition, in addition to thenoble-metal-carrying silicon carbide particles 1, other catalyticparticles, inorganic particles, and metal particles may be contained.

Porosity of the porous layer is preferably 40% to 90%, and morepreferably 50% to 80%. When the porosity is less than 40%, there is aconcern that sufficient gas diffusion does not occur in the poroussubstance, and thus the catalyst may not operate effectively. Inaddition, when the porosity exceeds 90%, the mechanical strength of theporous layer decrease, and thus there is a concern that deformation ofthe porous layer itself or peeling-off from the base material occurs,and thus the catalytic operation may decrease.

FIG. 2 is a schematic view illustrating a porous film 20 of thisembodiment. FIG. 3 is a scanning electron microscope (SEM) imageillustrating an example of the porous film of this embodiment. Withregard to the porous film 20, a plurality of kinds ofnoble-metal-carrying silicon carbide particles 21 a and 21 b andsurface-coated silicon carbide particles 25 agglomerate and the entiretyof these particles form a porous film shape.

Here, in the noble-metal-carrying silicon carbide particle 21 a, a noblemetal particle 23 is carried on a surface of a silicon carbide particle22, and the noble metal particle 23 is carried on the surface of thesilicon carbide particle 22 in a state of being coated with an oxidelayer 24.

In addition, in the noble-metal-carrying silicon carbide particle 21 b,two noble metal particles 23 are carried on the surface of the siliconcarbide particle 22, and the noble metal particles 23 are carried on thesurface of the silicon carbide particle 22 in a state of being coatedwith the oxide layer 24.

The number of the noble metal particles 23, which are carried by thenoble-metal-carrying silicon carbide particle may be 3 or more than 1 or2.

On the other hand, in each of the surface-coated silicon carbideparticles 25, the surface of the silicon carbide particle 22 is coatedwith the oxide layer 24, and the noble metal particle 23 is not carriedon the surface of the silicon carbide particle 22.

As described above, the porous film 20 may be constituted by a mixtureof the noble-metal-carrying silicon carbide particles 21 a and 21 b, andthe surface-coated silicon carbide particle 25 on which the noble metalparticle 23 is not carried. The porous film 20 may be constituted byonly the noble-metal-carrying silicon carbide particles 21 a and 21 b,or may contain a noble-metal-carrying silicon carbide particle on whichthree or more noble metal particles 23 are carried.

Further, the noble-metal-carrying silicon carbide particle or thesurface-coated silicon carbide particle may contain at least one kind ofelement selected from group 3 elements to group 14 elements such assilicon (Si), aluminum (Al), boron (B), zirconium (Zr), and titanium(Ti), or oxides thereof, carbides thereof, and nitride thereof asnecessary. These may be contained alone or in combinations thereof. Inaddition, if other components are contained, the percentage of siliconcarbide is preferably 80% by volume or more, and more preferably 90% byvolume or more.

When an exhaust gas discharged from an internal combustion engine flowsinto the porous layer 20 that contains the noble-metal-carrying siliconcarbide particle 1, the exhaust gas comes into contact with thenoble-metal-carrying silicon carbide particle 1, which constitutes thecatalyst, during flow, and thus carbon monoxide (CO), hydrocarbon (HC),nitrogen oxide (NO_(R)), particulate matter (PM), and the like which arecontained in the exhaust gas are oxidized and decomposed by thenoble-metal-carrying silicon carbide particles 21 a and 21 b.Specifically, CO is oxidized and decomposed into CO₂, HC is oxidized anddecomposed into H₂O and CO₂, NO_(x) is oxidized and decomposed into NO₂,and carbon that constitutes the PM is oxidized and decomposed to CO₂. Inaddition, the surface-coated silicon carbide particle 25 also has anoxidizing and decomposing ability due to the oxygen release abilityprovided to the oxide layer 24 on the surface, and thus the oxidizationand decomposition of CO, HC, NO_(R), and PM by the particle itself, oran auxiliary operation for the oxidizing and decomposing operationprovided to the noble-metal-carrying silicon carbide particles 21 a and21 b occurs.

Then, NO₂ is reduced to N₂, and then a purified gas, from which harmfulcomponents or the particulate matter is removed after decomposition, isdischarged to the air.

[Method of Manufacturing Exhaust Purification Catalyst and ExhaustEmission Control Device for Internal Combustion Engine]

The method of manufacturing the exhaust purification catalyst and theexhaust emission control device for an internal combustion engineaccording to this embodiment will be described.

In this embodiment, a method in which, first, the exhaust purificationcatalyst is manufactured and the exhaust emission control device ismanufactured using the exhaust purification catalyst that is obtainedmay be employed. On the other hand, a method in which the exhaustpurification catalyst is simultaneously prepared during manufacturing ofthe exhaust emission control device is also possible. Therefore, in thefollowing description, the manufacturing method is divided intorespective processes, and a possible process sequence is illustratedfirst, and then the details of the respective processes are described.

The method of manufacturing the exhaust purification catalyst and theexhaust emission control device for an internal combustion engine ofthis embodiment includes a mother material preparing process ofpreparing silicon carbide particles as a mother material ([A] process),a noble metal particle carrying process of carrying the noble metalparticle on the silicon carbide particle ([B] process), and an oxidelayer forming process of forming the oxide layer on the surface of thesilicon carbide particle ([C] process). In addition, in the method ofmanufacturing the exhaust emission control device for an internalcombustion engine, a process of carrying the silicon carbide particle ona base material that constitutes the exhaust emission control device([D] process) is also included.

A sequence of these four processes of [A] to [D] has the followingrestriction. First, it is necessary for the mother material preparingprocess [A] to be performed first. Next, it is necessary for the oxidelayer forming process [C] to be performed with respect to the siliconcarbide particle on which the noble metal particle is carried (notperformed with respect to a silicon carbide particle alone). The reasonfor the restriction is as follows. Even when the noble metal particle iscarried after forming the oxide layer in the silicon carbide particle,sufficient oxidizing catalytic characteristics cannot be obtained. Next,the carrying process [D] on the base material is not necessary in theprocess of manufacturing the exhaust purification catalyst. There arethree restrictions, and thus a sequence may be arbitrarily selected aslong as these restrictions are satisfied.

In addition, the noble metal particle carrying process [B] and thecarrying process [D] on the base material have similar processes such asdissolution or dispersion in a solvent (dispersion medium), drying andremoval of the solvent (dispersion medium), and a heat treatment.

Accordingly, when manufacturing the exhaust emission control device, thenoble metal particle carrying process [B] and the carrying process [D]on the base material may be simultaneously performed.

As described above, in the method of manufacturing the exhaustpurification catalyst and the exhaust emission control device for aninternal combustion engine of this embodiment, the following processsequence may be selected. In addition, it is assumed that a case ofperforming a subsequent process after an arbitrary process is indicatedby “→ (right arrow)”, and a case of simultaneously performing twoprocesses is indicated by “=”.

First, the method of manufacturing the exhaust purification catalyst isperformed in the following process sequence.

[A]→[B]→[C]  (1-1):

In this method, the surface oxide layer is formed on the silicon carbideparticle after carrying the noble metal particle on the silicon carbideparticle. This one kind of process sequence only satisfies theabove-described restrictions.

Next, the method of manufacturing the exhaust emission control device isperformed by the following four kinds of process sequences.

[A]→[B]→[C]→[D]  (2-1):

In this method, after manufacturing the exhaust purification catalyst(the silicon carbide particle on which the noble metal particle iscarried and the surface oxide layer is formed), the exhaust purificationcatalyst is carried on the base material.

[A]→[B]→[D]→[C]  (2-2):

In this method, after forming the silicon carbide particle on which thenoble metal particle is carried, and carrying this silicon carbideparticle on the base material, the surface oxide layer is formed.

[A]→[D]→[B]→[C]  (2-3):

In this method, after carrying the silicon carbide particle, which isthe mother material, on the base material, the noble metal particle iscarried on the silicon carbide particle that is carried, and then thesurface oxide layer is formed.

[A]→[B]=[D]→[C]  (2-4):

In this method, after simultaneously performing the carrying of thenoble metal particle on the silicon carbide particle that becomes themother material, and the carrying of the silicon carbide particle thatbecomes the mother material on the base material, the surface oxidelayer is formed on the noble-metal-particle-carrying porous siliconcarbide layer that is obtained.

Next, the respective processes will be described in detail.

“Mother Material Preparing Process”

In this process, the silicon carbide particle that becomes the mothermaterial is prepared.

An average primary particle size of the silicon carbide particle may beselected depending on characteristics of the exhaust purificationcatalyst and the exhaust emission control device, which are required,but a range of 0.01 μm to 5 μm is preferable. When the silicon carbideparticle is a nanometer-sized particle, the silicon carbide particle maybe obtained using the above-described methods, that is, the thermalplasma method, the silica precursor baking method, and the like. Whenthe silicon carbide particle is a particle having a size distributionfrom submicron to micron (micrometer), the silicon carbide particle maybe obtained using the Acheson process, the silica reduction method, thesilicon carbonization method, and the like.

“Noble Metal Particle Carrying Process”

In this process, the noble metal particle is carried on the surface ofthe silicon carbide particle that becomes the mother material, or on thesurface of silicon carbide particle which is carried on the basematerial that constitutes the exhaust emission control device and inwhich the porous silicon carbide particle layer is formed, therebyforming noble-metal-carrying silicon carbide particles or a porous layerformed from the noble-metal-carrying silicon carbide particles.

First, a solution in which noble metal salts as sources of the noblemetal are dissolved, or a dispersion liquid in which noble metalcompound fine particles are dispersed is prepared. As a solvent ordispersion medium, water is preferable. However, in a case where thenoble metal sources are decomposed in water and precipitate, an organicsolvent may be used. As the organic solvent, a polar solvent ispreferable, and alcohols, ketones, and the like are appropriately used.

Next, the silicon carbide particles are immersed and dispersed in thesolution or the dispersion liquid, or the porous silicon carbideparticle layer (the base material on which the silicon carbide particleis carried) is immersed in the solution or the dispersion liquid and isdried at a temperature of approximately 60° C. to 250° C. to remove thewater or the dispersion medium. According to this, the noble metal saltsor the noble metal compound fine particles may be attached to thesurface of the silicon carbide particles (including particles formingthe porous layer).

Next, the porous layer or the silicon carbide particles to which thenoble metal salts or the noble metal compound fine particles areattached is subjected to a heat treatment in a reducing atmosphereincluding hydrogen, carbon monoxide, and the like, or in an inertatmosphere such as nitrogen, argon, neon, xenon, and the like, therebyreducing and decomposing the noble metal salts or the noble metalcompound to form the noble metal fine particle. A heat treatmenttemperature or a heat treatment time may be appropriately selecteddepending on the kinds of the noble metal sources, atmosphericconditions, and the like. However, typically, the heat temperature isset to be in a range of 500° C. to 1500° C., and the heat treatment timeis set to be in a range of 10 minutes to 24 hours. In addition, atemperature higher than necessary and time longer than necessary maycause sintering of the silicon carbide particles or sintering of thenoble metal particles that are generated, and thus are not preferable.According to the above-described processes, it is possible to obtain thesilicon carbide particles in which the noble metal fine particles arecarried on the surface, or the porous layer formed from the siliconcarbide particles which are carried on the base material and in whichthe noble metal fine particles are carried on a surface.

Examples of the noble metal sources, which are used in the processes,include salts or compounds which contain one or two or more kinds ofnoble metal elements selected from a group consisting of platinum (Pt),gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd),osmium (Os), and iridium (Ir) which are noble metal particles, forexample, chlorides, sulfates, nitrates, organic acid salts, complex(complex salts), hydroxides, and the like.

“Oxide Layer Forming Process”

In this process, the silicon carbide particle on which the noble metalparticle is carried, or the porous silicon carbide particle layer whichis carried on the base material and on which the noble metal particle iscarried is subjected to an oxidizing treatment to form the oxide layeron the surface of the noble-metal-particle-carrying silicon carbideparticle, whereby the exhaust purification catalyst (silicon carbideparticle which carries the noble metal particle and on which the surfaceoxide layer is formed), or the porous layer formed from the exhaustpurification catalyst is formed.

With regard to the oxidizing treatment, the noble-metal-carrying siliconcarbide particle itself, or the porous silicon carbide particle layerwhich is carried on the base material and on which the noble metalparticle is carried, that is, the base material including a layer inwhich noble-metal-carrying silicon carbide particles are partiallysintered is subjected to the oxidizing treatment in an oxidizingatmosphere such as air and oxygen at a temperature of 600° C. to 1000°C. and preferably 650° C. to 900° C. for 0.5 hours to 36 hours andpreferably 4 hours to 12 hours, thereby forming an oxide layer on thesurface of the silicon carbide particle.

According to the present embodiment, it is possible to form the siliconcarbide particle, that is, the exhaust purification catalyst, in whichthe noble metal particle is carried on a surface and the noble particleis coated with the oxide layer, or the porous layer, that is, theexhaust emission control device in which the silicon carbide particle iscarried on the base material in a state in which the noble metalparticle is carried on the surface of the silicon carbide particle andthe noble metal particles are coated with the oxide layer. As describedabove, when the oxide layer is formed under the above-describedconditions, and the average primary particle size of the noble metalparticle is 50 nm or less, it is possible to form a catalyst particlehaving high catalytic activity in which the noble metal particle iscompletely coated with the oxide layer without being protruded from theoxide layer.

“Carrying Process on Base Material”

This carrying process on the base material includes a process ofdispersing any one of the silicon carbide particles as the mothermaterial, the noble-metal-particle-carrying silicon carbide particles,and the silicon carbide particles on which the noble metal particle iscarried and in which the surface oxide layer is formed in the dispersionmedium to prepare a silicon carbide particle dispersion liquid, and aprocess of applying (coating) the silicon carbide particle dispersionliquid on the base material that constitutes the exhaust emissioncontrol device, performing drying, and partially sintering the siliconcarbide particles to form a porous layer.

First, any one of the silicon carbide particles, thenoble-metal-particle-carrying silicon carbide particles, and the siliconcarbide particles, on which the noble metal particle is carried and inwhich the surface oxide layer is formed, is dispersed in the dispersionmedium to prepare a silicon carbide particle dispersion liquid.

As the dispersion medium, any dispersion medium may be used as long asthis dispersion medium can uniformly disperse silicon carbide particles,and water or an organic solvent is appropriately used. In addition, anelementary substance of a polymeric monomer or an oligomer or a mixtureof these may be used as necessary.

Examples of the organic solvent that is appropriately used includealcohols such as methanol, ethanol, 1-propanol, 2-propanol, diacetonealcohol, furfuryl alcohol, ethylene glycol, and hexylene glycol; esterssuch as acetic acid methyl ester and acetic acid ethyl ester; etherssuch as diethyl ether, ethylene glycol monomethyl ether (methylcellosolve), ethylene glycol monoethyl ether (ethyl cellosolve),ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, dioxane, andtetrahydrofuran; ketones such as acetone, methyl ethyl ketone,acetylacetone, and acetoacetic ester; acid amides such asN,N-dimethylformamide; aromatic hydrocarbon such as toluene and xylene;and the like. These solvents may be used alone or two or more kindsthereof may be mixed and used.

As the polymeric monomer, an acryl-based monomer or methacryl-basedmonomer such as methyl acrylate and methyl methacrylate, an epoxy-basedmonomer, and the like may be used. In addition, as the oligomer,urethane acrylate-based oligomer, epoxy acrylate-based oligomer,acrylate-based oligomer, and the like may be used.

A dispersing agent (surface treatment agent), a surfactant, apreservative, a stabilizing agent, a defoaming agent, a leveling agent,and the like may be appropriately added to the dispersion liquid tosecure dispersion stability or to improve application property. An addedamount of the dispersing agent, the surfactant, the preservative, thestabilizing agent, the defoaming agent, and the leveling agent is notparticularly limited, and the addition may be performed depending on anaddition purpose.

As a device that is applicable to a carrier preparing process, forexample, a kneader, a roll mill, a pin mill, a sand mill, a ball mill, aplanetary ball mill, and the like may be exemplified as a device capableof dispersing the silicon carbide particle in the dispersion medium, butthe sand mill, the ball mill, and the planetary ball mill, and the likeare preferable to disperse the silicon carbide particle in thedispersion medium using a dispersive medium.

In addition, examples of the dispersive medium such as a ball include aresin-covered body including a metal core formed from steel, lead, andthe like is formed, a sintered body of an inorganic oxide such asalumina, zirconia, silica, and titania, a sintered body of a nitridesuch as silicon nitride, a sintered body of a silicide such as siliconcarbide, glass such as soda glass, lead glass, and high specific gravityglass, but as the dispersive medium that is used in this embodiment,from the viewpoints of mixing and dispersing efficiency, zirconia havingspecific gravity 6 or more, the resin-covered body including the coreformed from steel, and the like are preferable.

Next, the silicon carbide particle dispersion liquid is applied to thebase material that constitutes the exhaust emission control device, forexample, a base material having a honeycomb structure (hereinafter, alsoreferred to as a “honeycomb base material”), and is dried to form acoated film (applied and dried layer) on the base material.

Here, the “honeycomb structure” in this embodiment represents astructure in which a plurality of gas flow passages which are divided bya partition wall and are disposed in parallel with each other, and ofwhich both ends have opening are formed in the base material. An exhaustgas discharged from an internal combustion engine flows into from oneend side, the exhaust gas comes into contact with the catalyst disposedon the partition wall during passing through the gas flow passage and ispurified, and the purified exhaust gas is discharged from the other endside.

Here, the silicon carbide particle dispersion liquid is applied on thebase material such as the honeycomb base material by a bar coat method,a slip cast method, a wash coat method, and the like, and is dried toform a coated film on the base material. In addition, the coated filmmay be formed on the base material by immersing the base material suchas the honeycomb base material in the silicon carbide particledispersion liquid, and by pulling up and drying the base material.

In addition, the drying, that is, the removal of the water or organicsolvent may be performed in the atmospheric atmosphere at a temperatureof approximately 100° C. to 250° C.

Next, the base material such as the honeycomb base material on which thecoated film containing the silicon carbide particle is formed issubjected to a heat treatment under an inert atmosphere such asnitrogen, argon, neon, and xenon, or in a reducing atmosphere such ashydrogen and carbon monoxide at a temperature of 500° C. to 1500° C.,and preferably a temperature of 600° C. to 1100° C.

According to this heat treatment, the dispersion medium in the coatedfilm, that is, the water or the organic solvent that remains, or thepolymeric monomer or the oligomer, or an additive such as the dispersingagent, the surfactant, the preservative, the stabilizing agent, thedefoaming agent, and the leveling agent in the coated film is scattered,and the silicon carbide particles are partially sintered to form a neckportion in which the particles are bonded to each other, whereby aporous film bonded in a porous substance shape is formed.

In addition, the drying of the dispersion liquid may be integrated witha heat treatment process as a previous stage of the heat treatmentprocess.

In addition, said heat treatment process may be omitted as long as theoxide layer may be formed on the surface of the silicon carbide particleand silicon carbide particles may be partially sintered by adjustingheat treatment conditions in the oxide layer forming process.

(Case of Simultaneously Performing Noble Metal Particle Carrying Processand Porous Film Forming Process)

The noble metal particle carrying process and the porous film formingprocess are similar processes. Accordingly, both of the processes may besimultaneously performed in parallel with each other in the method ofmanufacturing the exhaust emission control device used in an internalcombustion engine.

First, the process of preparing the solution in which the noble metalsalts as the source of the noble metal are dissolved, or the dispersionliquid in which the noble metal compound fine particles are dispersed inthe noble metal particle carrying process, and the process of preparingthe dispersion medium in the porous film forming process are similar toeach other. In addition, the process of immersing and dispersing thesilicon carbide particle in the solution or the dispersion liquid in thenoble metal particle carrying process and the process of preparing thesilicon carbide particle dispersion liquid in the porous film formingprocess are similar to each other.

Accordingly, these processes may be collectively performed in a singleprocess by dissolving or dispersing the noble metal sources in thesilicon carbide particle dispersion medium in the porous film formingprocess. In addition, as the dispersion medium, it is necessary toselect a substance in which the noble metal sources can be easilydissolved or dispersed, and particularly, water may be used whenappropriate. On the other hand, in a case of using the organic solventand the like, the noble metal sources that are easily dispersed in theorganic solvent may be selected.

Next, the porous film forming process includes a process of applying(coating) the silicon carbide particle dispersion liquid on thepartition wall which constitutes the filter base body and is formed fromthe porous substance, but the noble metal particle carrying process doesnot include a corresponding process.

Next, the process of removing the water or the dispersion medium, whichis a solvent, for drying in the noble metal particle carrying process,and the process of drying the partition wall which is formed from theporous substance and to which the silicon carbide particle dispersionliquid is applied in the porous film forming process are similar to eachother.

These both processes are drying processes and are substantially the sameprocesses, and thus these processes may be performed in a singleprocess. A drying temperature may be set to be in a range that is commonto both processes, that is, a temperature of approximately 100° C. to250° C.

Next, the process of forming the noble metal fine particle by performingthe heat treatment in the reducing atmosphere or the inert atmosphere inthe noble metal particle carrying process, and the process of formingthe porous film by subjecting the partition wall in which the appliedand dried film is formed and which is formed form the porous substanceto the heat treatment in the reducing atmosphere or the inert atmosphereto allow the partition wall to be partially sintered in the porous filmforming process are similarly to each other.

Since the both processes are substantially the same processes from theviewpoint of performing the heat treatment in the reducing atmosphere orthe inert atmosphere, these processes may be performed in a singleprocess.

A heat treatment temperature or a heat treatment time may be selected ina range common to both processes in consideration of noble metalparticle generation conditions or porous film forming conditions.

When performing the above-described processes, that is, the process ofdissolving or dispersing the noble metal sources in the dispersionmedium, the process of dispersing the silicon carbide particle in thedispersion medium, the process of applying the silicon carbide particledispersion liquid that is obtained to the partition wall whichconstitutes the filter base body and is formed from the poroussubstance, the process of drying the coated film that is obtained, andthe process of subjecting the coated film that is obtained to the heattreatment, the noble metal particle carrying process is integrated inthe porous film forming process. Accordingly, it is possible tosimultaneously perform the noble metal particle carrying process and theporous film forming process in parallel with each other as an integratedprocess.

According to the above-described configuration, it is possible toprepare the exhaust emission control device for an internal combustionengine of this embodiment.

As described above, according to the exhaust purification catalyst ofthis embodiment, the noble metal particle is carried on the surface ofthe silicon carbide particle, and the noble metal particles are carriedin a state of being coated with the oxide layer, and thus the noblemetal fine particles are uniformly carried on the surface of the siliconcarbide particle. In addition, the noble metal particles are carried ina state of being coated with the oxide layer. Accordingly, it ispossible to secure an effective catalytic active site. In addition, thenoble metal particle is present as a fine particle which is highlydispersed (has large specific surface area), and thus it is possible toexhibit and maintain high catalytic activity at a low temperature, forexample, 200° C. or lower without increasing the amount of noble metalparticles.

In addition, the fine silicon carbide particle itself has durability ata high temperature, and the noble metal particle is carried in a stateof being coated with the oxide layer, and thus even in ahigh-temperature environment, it is possible to maintain the same highcatalytic activity as that at the low temperature.

According to the above-described configuration, the high catalyticactivity at a low temperature and the high durability at a hightemperature are compatible with each other. As a result, it is possibleto maintain high durability while maintaining high catalytic activityover a low-temperature region to a high-temperature region.

In addition, typically, grain growth of the fine noble metal particleprogresses at a relatively low temperature. However, since the noblemetal particles of this embodiment are coated with the oxide layer onthe surface of the silicon carbide particle, the oxide layer and thenoble metal particle can bond together, and thus even at a hightemperature, migration of the noble metal particles on the carrier canbe suppressed, and thus sintering (grain growth) of the noble metalparticles can be prevented.

Particularly, since “silicon carboxide” which tends to form a bondingwith the noble metal particles is contained, the effect of preventingsintering of the noble metal particles further increases.

In addition, it is known that the melting point of a fine noble metalparticles is lowered. However, melting of the noble metal particles canbe suppressed due to a chemical bond with the oxide layer, particularly,the silicon carboxide.

In addition, the silicon carbide is stable even at high temperatures,and thus grain growth is less likely to occur in comparison to ceramicsformed from a metal oxide such as alumina, ceria, and zirconia, or ametal composite oxide having a perovskite structure expressed by GeneralFormula ABO₃ (provided that, A and B represent metal elements) which hasbeen used in the related art. According to this, even after beingexposed to a high temperature in a manufacturing process or in use, itis possible to maintain a distance between noble metal particles, and asa result, it is possible to prevent contact between the noble metalparticles. Accordingly, sintering (grain growth) is less likely tooccur.

According to the exhaust emission control device for an internalcombustion engine of this embodiment, the exhaust purification catalystof this embodiment is at least one catalyst that is disposed in theexhaust passage of the internal combustion engine. Accordingly, thecatalytic activity at a low temperature is high, and even when beingexposed to a high temperature, the catalytic activity does not decrease,and thus high durability can be maintained. Accordingly, it is possibleto effectively oxidize, decompose, and remove carbon monoxide (CO),hydrocarbon (HC), nitrogen oxide (NO_(x)), particulate matter (PM), andthe like which are contained in an exhaust gas discharged from aninternal combustion engine, and thus it is possible to obtain an exhaustemission control device for an internal combustion engine, which hasfurther higher reliability.

EXAMPLES

Hereinafter, the invention will be described in detail with reference toExamples and Comparative Examples, but the invention is not limited byExamples.

[Each Measurement and Evaluation Method]

First, a description will be made with respect to an evaluation methodin the exhaust purification catalysts and exhaust emission controldevices for an internal combustion engine of Examples and ComparativeExamples.

(1) Composition of Oxide Layer

The porous layer containing the catalyst was cut for each honeycomb basematerial and was measured by using an X-ray photoelectric analyzer(XPS/ESCA) (Sigma Probe, manufactured by VG-Scientific Co.) to performsurface composition analysis with respect to a surface of a SiC particlethat forms the porous layer.

(2) Carried State of Noble Metal Particle and Average Primary ParticleSize

The porous layer containing the catalyst was cut for each honeycomb basematerial, and was observed by using a field emission transmissionelectron microscope (FE-TEM) (JEM-2100F, manufactured by JapanElectronic Co., Ltd) to evaluate a carried state of the noble metalparticles.

In addition, with regard to the average primary particle size of thenoble metal particles, the primary particle sizes of 500 noble metalparticles, which were randomly selected from an observed image obtainedin the same manner, were measured, and the average value thereof was setto the average primary particle size of the noble metal particles.

(3) Specific Surface Area

The porous layer containing the catalyst was cut for each honeycomb basematerial, and a specific surface area was measured by using a BETspecific surface area measurement device (BELSORP-mini, manufactured byJapan BEL Co., Ltd.), and a value, which was obtained by subtracting aspecific surface area corresponding to the substrate weight that wasmeasured separately from the measured specific surface area, was thespecific surface area of the catalytic layer.

(4) Average Porosity of Porous Film

The porous layer containing the catalyst was cut for each honeycomb basematerial, and average porosity of a porous layer portion containing thecatalyst was measured by using a mercury porosimeter (Pore Master 60GT,manufactured by Quantachrome Co., Ltd.).

(5) CO Purification Temperature and HC Purification Temperature

The honeycomb base material (hereinafter, also referred to as“porous-layer-formed honeycomb base material”), in which the porouslayer containing the catalyst was formed, was mounted in the exhaustemission control device (exhaust purification catalyst evaluationdevice), and the temperature of the porous-layer-formed honeycomb basematerial was increased while a simulated exhaust gas was introduced toflow through the exhaust emission control device. Then, the componentsof the simulated exhaust gas after flowing through the exhaust emissioncontrol device were measured to measure a purification rate of carbonmonoxide (CO) and hydrocarbon (HC). Here, a temperature (T50) of afilter base body when 50% of CO or HC was purified, that is, atemperature when an amount of CO or HC in the simulated exhaust gasafter passing through the exhaust emission control device became a halfof an amount CO or HC in the simulated exhaust gas that was introducedwas an index.

FIG. 4 is a schematic view of the exhaust emission control device thatwas used in this test. In the exhaust emission control device 11, aporous-layer-formed honeycomb base material 13 as a test specimen wasdisposed inside of a tubular exhaust passage 12, a bottle 14, whichstores the simulated exhaust gas G shown in Table 1, was disposed on anupstream side of the exhaust passage 12, and a cylindrical heatingfurnace 15 was provided to surround the exhaust passage 12. The heatingfurnace 15 was configured to control the inside of the exhaust passage12 using a control device (not shown) at a desired temperature. Here, asthe porous-layer-formed honeycomb base material 13, a honeycomb basematerial, which had a volume of 29 cm³ and in which SV was 28000/mesh,was used.

Table 1 shows the components of the simulated exhaust gas that was usedin this measurement.

A temperature condition during measurement was set to be in atemperature-lowering condition from 500° C. by 17° C./minute, and a flowrate (special speed) of the simulated exhaust gas G was set to 13.5L/minute.

In addition, a temperature inside the furnace was measured at a positionon a downstream side of the porous-layer-formed honeycomb base material13 by 10 mm, and this temperature was used as the temperature of theporous-layer-formed honeycomb base material.

TABLE 1 Kinds of gas Content ratio O₂ 6% by volume CO₂ 10% by volumeHC(C₃H₆) 500 ppm (167 ppm) CO 1000 ppm NO 200 ppm H₂O 7% by volume N₂Remainder

(6) Evaluation of Heat Resistance

The honeycomb base material in which the catalyst-containing porouslayer was formed was subjected to a heat treatment in the air at 700° C.for 30 hours.

With respect to the porous-layer-formed honeycomb base material afterthe heat treatment, a specific surface area, a CO purificationtemperature and a HC purification temperature were measured by theabove-described method, and the comparison with results before the heattreatment was made to evaluate the heat resistance.

Example 1

15 g of silicon carbide particles having an average primary particlesize of 0.035 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.01 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 10.0%by volume, 87.5% by volume, and 2.5% by volume. Next, theplatinum-salt-carrying silicon carbide particles and water were mixedwith a ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 12 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Example 1.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from theplatinum-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions.

First step: temperature=980° C., retention time=80 minutes, andatmosphere=argon

Second step: temperature=730° C., retention time=360 minutes, andatmosphere=air

Through this heat treatment, formation of a porous layer by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a honeycomb base material inwhich a porous layer containing the catalyst of Example 1 was formed.

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 3 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer, which was formed from an amorphous compound (SiO_(x), providedthat, 0<x≦3) containing silicon and oxygen in combination and anamorphous compound (SiO_(y)C_(z) provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the platinum fine particlewas coated with the oxide layer. In addition, in a field-emissiontransmission electron microscope image (FE-TEM image) of the exhaustpurification catalyst that was obtained, a crystal lattice image was notrecognized in the oxide layer. From this observation, it was discoveredthat the oxide layer was an amorphous substance.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 39 m²/g and 75%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 173° C. and 186° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were36 m²/g, 183° C., and 199° C., respectively.

These results are collectively shown in Table 2.

Example 2

15 g of silicon carbide particles having an average primary particlesize of 0.015 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.05 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles weresubjected to a heat treatment under the following conditions. Inaddition, a first step corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=600° C., retention time=90 minutes, andatmosphere=argon

Third step: temperature=700° C., retention time=60 minutes, andatmosphere=air

Through this heat treatment, formation of platinum (noble metal) fineparticles by reduction and decomposition of dinitro-platinate carried onthe surface of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebyforming noble-metal-carrying silicon carbide particles in which theoxide layer was formed.

Next, the noble-metal-carrying silicon carbide particles in which theoxide layer was formed, water, and gelatin used as a gelating agent wereweighed in content ratios of 6.0% by volume, 93.0% by volume, and 1.0%by volume. Next, the noble-metal-carrying silicon carbide particles inwhich the oxide layer was formed and water were mixed with a ball millusing a resin ball including an iron core at a rotation speed of 220 rpmfor 48 hours to obtain a dispersion liquid. Then, the gelatin was addedto the dispersion liquid that was obtained and mixing was performed for20 minutes to obtain a silicon carbide particle dispersion liquid(application liquid) of Example 2.

Next, a honeycomb-structured base material formed from aluminum titanatewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer, which was formed from thenoble-metal-carrying silicon carbide particles in which the oxide layerwas formed, on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions.

First step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Second step: temperature=700° C., retention time=12 hours, andatmosphere=air

Through this heat treatment, formation of a porous layer by partialsintering of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebypreparing a porous layer that contains the catalyst of Example 2.

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 10 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer, which was formed from an amorphous compound (SiO_(x), providedthat, 0<x≦3) containing silicon and oxygen in combination and anamorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the platinum fine particlewas coated with the oxide layer. In addition, as is the case withExample 1, from the FE-TEM image, it was discovered that the oxide layerwas an amorphous substance.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 85 m²/g and 89%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 169° C. and 179° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were81 m²/g, 178° C., and 192° C., respectively.

These results are collectively shown in Table 2.

Example 3

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining slurry.

Silicon carbide particles having an average primary particle size of 0.8μm were added to the slurry that was obtained, thereby preparing a mixedliquid in which an amount of platinum was adjusted to be 0.01 g on thebasis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 25.5% byvolume, the content ratio of water became 72.0% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 3. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 3.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from theplatinum-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions.

First step: temperature=900° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=600° C., retention time=480 minutes, andatmosphere=air

Through this heat treatment, formation of a porous layer by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a porous layer thatcontained the catalyst of Example 3.

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 10 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer, which was formed from an amorphous compound (SiO_(x), providedthat, 0<x≦3) containing silicon and oxygen in combination and anamorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the platinum fineparticles were coated with the oxide layer.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 13 m²/g and 68%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 179° C. and 183° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were11 m²/g, 188° C., and 201° C., respectively.

These results are collectively shown in Table 2.

FIG. 5 is a field-emission transmission electron microscope image(FE-TEM image) of an exhaust purification catalyst of Example 3, andFIG. 6 is an explanatory view illustrating a structure of the exhaustpurification catalyst shown in FIG. 5.

In these drawings, the silicon carbide particles, the oxide film (oxidelayer) that is formed on the surface of the silicon carbide particles,and the platinum fine particle (noble metal fine particle) coated withthe oxide film (oxide layer) are shown.

In the FE-TEM image, a crystal lattice image was not recognized in theoxide layer. From this observation, it is discovered that the oxidelayer was an amorphous substance.

Example 4

15 g of silicon carbide particles having an average primary particlesize of 0.06 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of palladium nitrate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of 5.0μm were added to the slurry that was obtained, thereby preparing a mixedliquid in which an amount of palladium was adjusted to be 0.01 g on thebasis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 35.0% byvolume, the content ratio of water became 63.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 1.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 4. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 4.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from thepalladium-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions.

First step: temperature=850° C., retention time=240 minutes, andatmosphere=argon

Second step: temperature=800° C., retention time=360 minutes, andatmosphere=air

Through this heat treatment, formation of a porous layer by partialsintering of the silicon carbide particles, formation of palladium(noble metal) fine particles by reduction and decomposition of palladiumnitrate carried on the surface of the silicon carbide particles, andformation of a surface oxide layer of the silicon carbide particles wereperformed, thereby preparing a porous layer that contained the catalystof Example 4.

In the exhaust purification catalyst that was obtained, a palladium fineparticle having an average primary particle size of 20 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer, which was formed from an amorphous compound (SiO_(x), providedthat, 0<x≦3) containing silicon and oxygen in combination and anamorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the palladium fineparticle was coated with the oxide layer. In addition, as is the casewith Example 1, from the FE-TEM image, it was discovered that the oxidelayer was an amorphous substance.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 4 m²/g and 55%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 205° C. and 209° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were3 m²/g, 221° C., and 235° C., respectively.

These results are collectively shown in Table 2.

Example 5

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of10.0 μm were added to the slurry that was obtained, thereby preparing amixed liquid in which an amount of platinum was adjusted to be 0.005 gon the basis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 30.0% byvolume, the content ratio of water became 67.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 5. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 5.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from theplatinum-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions.

First step: temperature=1000° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=730° C., retention time=60 minutes, andatmosphere=air

Through this heat treatment, formation of a porous layer by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a porous layer thatcontained the catalyst of Example 5.

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 1 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer, which was formed from an amorphous compound (SiO_(x), providedthat, 0<x≦3) containing silicon and oxygen in combination and anamorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the platinum fine particlewas coated with the oxide layer. In addition, as is the case withExample 1, from the FE-TEM image, it was discovered that the oxide layerwas an amorphous substance.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 1 m²/g and 51%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 220° C. and 227° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were1 m²/g, 241° C., and 248° C., respectively.

These results are collectively shown in Table 2.

Comparative Example 1

15 g of silicon carbide particles having an average primary particlesize of 0.035 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.01 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 10.0%by volume, 87.5% by volume, and 2.5% by volume. Next, theplatinum-salt-carrying silicon carbide particles and water were mixedwith a ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 12 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Comparative Example 1.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from theplatinum-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions,thereby preparing a porous layer that contained the catalyst ofComparative Example 1.

First step: temperature=980° C., retention time=70 minutes, andatmosphere=argon

Second step: temperature=450° C., retention time=30 minutes, andatmosphere=air

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 1 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer was not formed on the surface of the silicon carbide particles,and thus the platinum fine particle was not also coated with the oxidelayer.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 38 m²/g and 73%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 283° C. and 291° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were36 m²/g, 300° C., and 302° C., respectively.

These results are collectively shown in Table 2.

Comparative Example 2

15 g of silicon carbide particles having an average primary particlesize of 0.015 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.05 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles weresubjected to a heat treatment under the following conditions. Throughthe heat treatment, formation of platinum (noble metal) fine particlesby reduction and decomposition of dinitro-platinate carried on thesurface of the silicon carbide particles was performed, thereby formingthe noble-metal-carrying silicon carbide particles. In addition, a firststep corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Third step: temperature=450° C., retention time=240 minutes, andatmosphere=air

Next, the noble-metal-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 26.0%by volume, 73.0% by volume, and 1.0% by volume. Next, thenoble-metal-carrying silicon carbide particles and water were mixed witha ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 48 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Comparative Example 2.

Next, a honeycomb-structured base material formed from aluminum titanatewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from thenoble-metal-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions,thereby preparing a porous layer that contained the catalyst ofComparative Example 2.

First step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Second step: temperature=550° C., retention time=120 minutes, andatmosphere=air

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 1.5 nm was carriedon the surface of the silicon carbide particles. In addition, an oxidelayer was not formed on the surface of the silicon carbide particles,and thus the platinum fine particle were also not coated with the oxidelayer.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 83 m²/g and 68%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 261° C. and 276° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were80 m²/g, 296° C., and 302° C., respectively.

These results are collectively shown in Table 2.

Comparative Example 3

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of10.0 μm were added to the slurry that was obtained, thereby preparing amixed liquid in which the amount of platinum was adjusted to be 0.005 gon the basis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 30.0% byvolume, the content ratio of water became 67.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofComparative Example 3. First, ammonium polycarboxylate as a surfactantwas added to the mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in the amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of ComparativeExample 3.

Next, a honeycomb-structured base material formed from cordierite wasimmersed in the silicon carbide particle dispersion liquid. Then, thebase material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried layer formed from theplatinum-salt-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried layer was formedwas subjected to a heat treatment under the following conditions,thereby preparing a porous layer that contained the catalyst ofComparative Example 3.

First step: temperature=1000° C., retention time=360 minutes, andatmosphere=argon

Second step: temperature=500° C., retention time=360 minutes, andatmosphere=air

In the exhaust purification catalyst that was obtained, a platinum fineparticle having an average primary particle size of 3 nm was carried onthe surface of the silicon carbide particles. In addition, an oxidelayer was not formed on the surface of the silicon carbide particles,and thus the platinum fine particle was not also coated with the oxidelayer.

The specific surface area and the average porosity of the porous layercontaining the catalyst were 1 m²/g and 51%, respectively.

In addition, the CO purification temperature and the HC purificationtemperature, which were measured using the exhaust emission controldevice, were 255° C. and 264° C., respectively.

Further, the specific surface area, the CO purification temperature, andthe HC purification temperature after the heat treatment at 700° C. were1 m²/g, 277° C., and 295° C., respectively.

These results are collectively shown in Table 2.

TABLE 2 Initial value After heat treatment at 700° C. Specific CO HCSpecific CO HC Noble surface area T50 T50 surface area T50 T50 Porositymetal Oxide layer (m²/g) (° C.) (° C.) (m²/g) (° C.) (° C.) (%) Example1 Pt SiO_(x) (0 < x ≦ 3), 39 173 186 36 183 199 75 SiO_(y)C_(z) (0 < y ≦3, 0 < z ≦ 3) Example 2 Pt SiO_(x) (0 < x ≦ 3), 85 169 179 81 178 192 89SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Example 3 Pt SiO_(x) (0 < x ≦ 3), 13179 183 11 188 201 68 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Example 4 PtSiO_(x) (0 < x ≦ 3), 4 205 209 3 221 235 55 SiO_(y)C_(z) (0 < y ≦ 3, 0 <z ≦ 3) Example 5 Pt SiO_(x) (0 < x ≦ 3), 1 220 227 1 241 248 51SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Comparative Pt Oxide film is notformed 38 283 291 36 300 302 73 Example 1 Comparative Pt Oxide film isnot formed 83 261 276 80 296 302 68 Example 2 Comparative Pt Oxide filmis not formed 13 255 264 11 277 295 51 Example 3

In the exhaust purification catalysts of Examples 1 to 5, ananometer-sized noble metal fine particle was carried on the surface ofthe silicon carbide particles, an oxide layer was formed on the surfaceof the silicon carbide particle, and the noble metal fine particle wascoated with the oxide layer.

In exhaust emission control devices using the exhaust purificationcatalysts of Examples 1 to 4 among these catalysts, the CO purificationtemperature was as low as 169° C. to 205° C., the HC purificationtemperature was as low as 179° C. to 209° C., and these purificationtemperatures did not depend on the specific surface area of the exhaustpurification catalysts. From these results, it was confirmed that asufficient purification effect on CO and HC was obtained.

Further, after the heat treatment at 700° C., the CO purificationtemperature was increased only by approximately 10° C. to 15° C., andthe HC purification temperature was increased only by approximately 10°C. to 25° C. in comparison a case before treatment. From these results,it was confirmed that a sufficient purification effect on CO and HC wasmaintained even after the heat treatment at 700° C.

Next, in an exhaust emission control device using the exhaustpurification catalyst of Example 5, the CO purification temperature was220° C., and the HC purification temperature was 227° C. From theseresults, it was discovered that these purification temperatures werelower in comparison to Comparative Example, but were higher incomparison to other Examples. In addition, after the heat treatment at700° C., the CO purification temperature was 241° C. and the HCpurification temperature was 248° C. From these results, it wasdiscovered that these temperatures were lower in comparison toComparative Examples, but a temperature rising rate was higher incomparison to other Examples.

The reason for the above result is considered as follows. Since lots ofsilicon carbide particles in Example 5 have an average primary particlesize of 10 μm, and thus the specific surface area of the porous layer isas low as 1 m², and as a result, the catalytic activity is lower incomparison to other Examples and the specific surface area after theheat treatment at 700° C. is small. Accordingly, sintering (graingrowth) between noble metal particles progresses.

That is, it was discovered that in Example 5, a sufficient purificationeffect on CO and HC was confirmed, but the effect was slightly lower incomparison to other Examples.

On the other hand, in the exhaust purification catalysts of ComparativeExamples 1 to 3, as is the case with Examples, a nanometer-sized noblemetal fine particle was carried on the surface of the silicon carbideparticle, but the oxide layer was not formed on the surface of thesilicon carbide particle, and as a result, the noble metal fineparticles were exposed without being coated with the oxide layer.

In the exhaust emission control device using the exhaust purificationcatalyst of Comparative Examples 1 to 3, the CO purification temperaturewas 255° C. to 283° C., and the HC purification temperature was 264° C.to 291° C. These purification temperatures were higher by approximately50° C. to 110° C. in comparison to Examples, and it was determined thata sufficient purification effect on CO and HC was not obtained.

Second Embodiment

Hereinafter, a second embodiment for carrying out an exhaust gaspurification filter of the invention will be described.

In addition, this embodiment makes a description in detail for easycomprehension of the gist of the invention, and does not limit theinvention unless otherwise stated.

[Exhaust Gas Purification Filter]

The exhaust gas purification filter of the second embodiment of theinvention will be described. Here, a DPF having a sealed-type honeycombstructure formed from ceramics, which is an exhaust gas purificationfilter used in a diesel engine for a vehicle, will be described as anexample.

The exhaust gas purification filter of this embodiment is an exhaust gaspurification filter which purifies an exhaust gas by allowingparticulate matter contained in the exhaust gas to pass through a filterbase body formed from a porous substance to trap the particulate matter.The filter base body includes a partition wall that is formed from aporous substance, an inflow-side gas flow passage which is formed by thepartition wall and in which an inflow-side end for an exhaust gas thatcontains particulate matter is opened, and an outflow-side gas flowpassage which is provided at a position different from that of theinflow-side gas flow passage of the filter base body and is formed bythe partition wall and in which an outflow-side end for the exhaust gasis opened. A porous film having a pore size smaller than that of thepartition wall is formed on a surface of the partition wall at least onan inflow-side gas flow passage side, and the porous film containssilicon carbide particles and noble metal fine particles, and the noblemetal particles, which are carried on the surface of the silicon carbideparticles, are carried in a state of being coated with an oxide layer.

FIG. 7 is a partially broken perspective view illustrating a DPF that isan example of the exhaust gas purification filter of an embodiment ofthe invention, and FIG. 8 is a cross-sectional view illustrating apartition wall structure of the DPF in a plane indicated by a symbol βin FIG. 7.

As shown in FIG. 7, the DPF 100 is an exhaust gas purification filterthat purifies the exhaust gas G by allowing the exhaust gas G to passthrough the filter base body 111 formed from cylindrical porous ceramicshaving a plurality of pores to trap particulate matter (PM) contained inthe exhaust gas G. In addition, it is not necessary for the DPF to havea cylindrical shape, and the DPF may have a shape such as a prism shapeor an elliptical cylinder shape.

Gas flow passages 112 are formed in the filter base body 111, and thegas flow passages 112 have a structure in which an upstream side end anda downstream side end are alternately closed when seen in a flowdirection of the exhaust gas G (longitudinal direction). That is, thegas flow passages 112 include an inflow cell (inflow-side gas flowpassage) 112A into which the exhaust gas G flows and in which an exhaustgas G inflow-side end is opened, the exhaust gas G containingparticulate matter, and an outflow cell (outflow-side gas flow passage)112B which is formed at a position different from the inflow cell 112Aand in which an exhaust gas G outflow-side end is opened. In each of thegas flow passages 112, a porous film 113 is formed on an inner wallsurface 112 a of the inflow cell 112A in which an exhaust gas Gupstream-side end (inflow-side end) is opened.

In addition, in both end faces of the filter base body 111 in an axialdirection, one end face α is an inflow face into which the exhaust gas Gcontaining particulate matter flows, and the other end face γ is anoutflow face from which a purified gas C after removing the particulatematter from the exhaust gas G is discharged.

A flow of the exhaust gas in the DPF 100 is illustrated in FIG. 8.

The exhaust gas G, which contains PM 130 and is introduced from theinflow face side, that is, the end face α side, flows into the inside ofthe DPF 100 from the inflow cell 112A of which an inflow face is opened,and passes through a partition wall 114 of the filter base body 111during flowing through the inside of the inflow cell 112A from the endface α side to the end face γ side. At this time, the PM 130 containedin the exhaust gas G is trapped and removed by the porous film 113 thatis provided on the inner wall surface 112 a (surface of a partition wall114 that constitutes the inflow cell 112A) of the inflow cell 112A. Thepurified gas C, from which the particulate matter 130 is removed, flowsthrough the inside of the outflow cell 112B from the end face α side tothe end face γ side, and is discharged to the outside of the filter froman opening end (end face γ) of the outflow cell 112B.

The filter base body 111 is a honeycomb structure formed fromheat-resistant porous ceramics such as silicon carbide, cordierite,aluminum titanate, and silicon nitride. The partition wall 114, whichextends along an axial direction that is a flow direction of the exhaustgas G, is formed in the filter base body 111, and axial hollow regionssurrounded by the partition wall 114 are the cell-shaped gas flowpassages 112 constituted by a plurality of the inflow cells 112A and aplurality of the outflow cells 112B.

Here, the “honeycomb structure” in this embodiment represents thefollowing structure. That is, the plurality of inflow cells 112A and theplurality of outflow cells 112B are formed in the filter base body 111to be parallel with each other, and the inflow cells 112A and theoutflow cells 112B are formed in such a manner that one side surroundsthe other side, and an opening end of one side and an opening end of theother side are opposite to each other.

Here, a cross-sectional shape in a direction perpendicular to an axialdirection of the inflow cell 112A and the outflow cell 112B, that is,the cross-sectional shape of the gas flow passage 112 is set to arectangular shape, but the cross-sectional shape is not limited to arectangular shape, and may be set to various shapes including apolygonal shape such as a hexagonal shape, a circular shape, anelliptical shape, and the like. In addition, in the gas flow passage 112formed in the vicinity of the outer periphery of the filter base body111, apart of the cross-sectional shape is an arc shape. However, thegas flow passage 112 having a cross-sectional shape, which confirms toan outer shape of the filter base body 111, is configured to dispose thegas flow passage 112 to the vicinity of the outer periphery of thefilter base body 111 without a gap.

The average pore size of the partition wall 114 formed from the porousceramics is preferably 5 μm to 50 μm. When the average pore size is lessthan 5 μm, pressure loss due to the partition wall 114 itself increases,and thus this pore size is not preferable. On the other hand, when theaverage pore size exceeds 50 μm, there is a concern that the strength ofthe partition wall 114 is not sufficient and it is difficult to form theporous film 113 on the partition wall 114, and thus this pore size isnot preferable.

In addition, the porous film 113 is a porous film which is formed on theinner wall surface 112 a of the inflow cell 112A and which has a poresize smaller than the pore size of the partition wall 114. The porousfilm 113 is formed on the inner wall surface 112 a of the inflow cell112A as an independent film without entering the inside of the pores ofthe porous ceramics that constitutes the partition wall 114 of thefilter base body 111. That is, the porous film 113 is formed on theinner wall surface 112 a of the inflow cell 112A in a state of intrudingonly to the inlet portion of the pores formed in the partition wall 114.In addition, the porous film 113 has a plurality of pores, and thesepores communicate with each other. As a result, the porous film 113 isformed from a filter-shaped porous substance having penetration holes.

Next, the porous film 113 will be described in detail.

FIG. 9 is a schematic view illustrating the porous film 113 of thisembodiment. With regard to porous film 113, a plurality of kinds ofnoble-metal-carrying silicon carbide particles 121 a and 121 b, andsurface-coated silicon carbide particles 125 agglomerate and theentirety of these particles form a porous film shape.

Here, in the noble-metal-carrying silicon carbide particle 121 a, anoble metal particle 123 is carried on a surface of a silicon carbideparticle 122, and the noble metal particle 123 is carried on the surfaceof the silicon carbide particle 122 in a state of being coated with anoxide layer 124.

In addition, in the noble-metal-carrying silicon carbide particle 121 b,two noble metal particles 123 are carried on the surface of the siliconcarbide particle 122, and the noble metal particles 123 are carried onthe surface of the silicon carbide particle 122 in a state of beingcoated with the oxide layer 124.

The number of the noble metal particles 123, which are carried by thenoble-metal-carrying silicon carbide particle may be 3 or more otherthan 1 or 2.

On the other hand, in each of the surface-coated silicon carbideparticles 125, the surface of the silicon carbide particle 122 is coatedwith the oxide layer 124, and the noble metal particle 123 is notcarried on the surface of the silicon carbide particle 122.

As described above, the porous film 113 may be constituted by a mixtureof the noble-metal-carrying silicon carbide particles 121 a and 121 b,and the surface-coated silicon carbide particle 125 on which the noblemetal particle 123 is not carried. The porous film 113 may beconstituted by only the noble-metal-carrying silicon carbide particles121 a and 121 b, or may contain a noble-metal-carrying silicon carbideparticle on which three or more noble metal particles 123 are carried.

Further, the noble-metal-carrying silicon carbide particle or thesurface-coated silicon carbide particle may contain at least one kind ofelement selected from group 3 elements to group 14 elements such assilicon (Si), aluminum (Al), boron (B), zirconium (Zr), and titanium(Ti), or oxides thereof, carbides thereof, and nitrides thereof asnecessary. These may be contained alone or in a combination manner. Inaddition, in a case of containing other components, the percentage ofsilicon carbide is preferably 80% by volume or more, and is morepreferably 90% by volume or more.

The noble-metal-carrying silicon carbide particles 121 a and 121 b serveas a combustion catalyst when combusting and removing trapped PM duringregeneration of the DPF 100, and has an effect of improving combustionefficiency such as lowering of a combustion temperature and shorteningof a combustion time.

In the related art, it is known that the noble metal particle has thecatalytic operation. In addition, the details will be described later,but it is known that the “oxides of silicon carbide” that constitutesthe oxide layer 124 exhibits oxygen release and also has the samecatalytic operation. Accordingly, when the noble metal particle 123 andthe oxide layer 124 are combined, it is considered that a relativelyhigher catalytic operation, that is, an effect such as further loweringof the combustion temperature or further shortening of the combustiontime is provided.

From the viewpoint of the combustion catalytic operation, even in a casewhere the surface-coated silicon carbide particle 131 on which the noblemetal particle is not carried is present, it is preferable that theoxide layer 124 be formed on the surface thereof.

With regard to the effect of having the catalytic effect, the porousfilm 113 may not have a complete film shape, and may include a porousagglomerate shape. In addition, the noble-metal-carrying silicon carbideparticles 121 a and 121 b may be disposed in the porous film 113 in adispersed manner, may be disposed in an agglomerated shape, or may bedisposed as a continuous film-shaped structure. In any case, the porousfilm 113 serves as a catalytic layer. In addition, the porous film 113may include not only the noble-metal-carrying silicon carbide particles121 a and 121 b, but also other catalytic particles, inorganicparticles, or metal particles.

On the other hand, in the exhaust gas purification filter (DPF 100) ofthis embodiment, it is preferable that PM be trapped by the porous film113. This is because when PM is trapped by the porous film 113, thetrapped PM and the noble-metal-carrying silicon carbide particles 121 aand 121 b that are PM combustion catalyst come into close contact witheach other, and thus the catalytic operation is exhibited effectively.

As described above, it is necessary for the porous film 113 to be formedon the inner wall surface 112 a of the inflow cell 112A as anindependent film without excessively entering the inside of the pores ofthe porous ceramics that constitutes the partition wall 114 of thefilter base body 111. In addition, it is necessary for the porous film113 to have an average pore size smaller than the average pore size ofthe partition wall 114. Specifically, the average pore size of theporous film 113 is preferably 0.05 μm to 3 μm, and more preferably 0.07μm to 2.5 μm. The reason for the limitation is as follows. In a casewhere the average pore size of the porous film 113 is less than 0.05 μm,when the exhaust gas that contains the particulate matter 130 flows intothe exhaust gas purification filter (DPF 100), pressure loss increases.On the other hand, in a case where the average pore size of the porousfilm 113 exceeds 3 μm, the pore size of the porous film 113 and the poresize of the partition wall 114 become substantially the same as eachother, and thus PM trapping properties of the porous film 113deteriorate. In addition to this deterioration, in a case of performinga regeneration treatment of the DPF 100, PM combustion efficiency is notimproved.

In addition, average porosity of the porous film 113 is preferably 50%to 90%, and more preferably 60% to 85%. The reason for the limitation isas follows. When the average porosity of the porous film 113 is lessthan 50%, the average porosity of the porous film 113 becomes equal toor less than the porosity of the partition wall 114, and thus anincrease in the pressure loss may be caused. On the other hand, when theaverage porosity of the porous film 113 exceeds 90%, there is a concernthat it is difficult to maintain the structure or strength of the porousfilm.

The thickness (film thickness) of the porous film 113 is preferably 60μm or less at a site planarly overlapping a vacant portion provided tothe partition wall 114 on the inner wall surface 112 a, and 5 μm to 60μm at a site planarly overlapping a solid portion of the partition wall114 on the inner wall surface 112 a.

Here, the “vacant portion” represents an opening in which a pore in thepartition wall 114 is opened on the inner wall surface 112 a, and thebase material that supports the porous film 113 is not present below theporous film 113. In addition, the “solid portion” represents a portionother than the vacant portion, that is, a portion in which the ceramicsmaterial that constitutes the partition wall 114 is present on the innerwall surface 112 a.

The reason of the limitation on the film thickness is as follows. Whenthe film thickness of the porous film 113 is in the above-describedrange, the pores are sufficiently present in the porous film, and thusan air flow on the porous film surface becomes substantially uniformwithout depending on the state of the partition wall 114. Accordingly,the pressure loss is reduced, and the PM 130 is uniformly trapped on theporous film 113, and thus satisfactory filter characteristics aremaintained.

Further, even in the filter regeneration treatment, a combustion gasthat combusts the trapped PM 130 may uniformly flow with respect to thePM 130 that is deposited, and thus the combustion efficiency isimproved.

However, when the thickness of the porous film 113 is less than 5 μm,the thickness of the porous film 113 is small, and thus the number ofpores in the porous film 113 substantially decreases. As a result, anair flow amount on the surface of the porous film 113 decreases at asite in which the porous film 113 planarly overlapping the solid portionof the partition wall 114, the air flow on the porous film surfacebecomes non-uniform. As a result, the pressure loss increases, or thetrapping of the PM 130 becomes non-uniform, and thus there is a concernthat the trapping efficiency decreases or the number of regenerationtreatments increases.

In addition, similarly, since the number of the pores in the porous film113 is small, even during the filter regeneration treatment, the flow ofthe combustion gas that combusts the trapped PM 130 becomes non-uniform,and thus there is a concern that the combustion efficiency of the PM 30may not be improved.

On the other hand, when the thickness of the porous film 113 exceeds 60μm, the pressure loss due to the porous film 113 becomes excessive, andthus there is a concern that an engine output decreases. Accordingly,this thickness range is not preferable.

The thickness of the porous film 113 is preferably 35 μm or less at thevacant portion and 7 μm to 35 μm at the solid portion, and morepreferably 30 μm or less at the vacant portion and 10 μm to 30 μm at thesolid portion.

Here, with regard to the silicon carbide particles 122 that constitutethe porous film 113, the lower limit of the average primary particlesize is preferably 0.01 μm. The reason for the limitation is as follows.When the average primary particle size of the silicon carbide particlesis less than 0.01 μm, the pore size of the porous film 113 that isobtained is too small, and thus there is a concern that the pressureloss in the DPF 100 increases. In addition, sintering (grain growth)between the silicon carbide particles progresses and the particle sizeincreases during use of the exhaust gas purification filter at a hightemperature. As a result, a catalytic active site decreases, and thusthis thickness range is not preferable. The lower limit is morepreferably 0.02 μm, and still more preferably 0.035 μm.

On the other hand, as conditions for defining the maximum value of theaverage primary particle size, there are two kinds of conditionsincluding a case where the characteristics of a filter are a mainfactor, and a case where combustion catalytic operation is a mainfactor. In the two cases, the maximum values are different from eachother.

First, in a case where the filter characteristics are the main factor,the maximum value of the average primary particle size is preferably 10μm. The reason for the limitation is as follows. When the maximum valueof the average primary particle size exceeds 10 μm, the pore size of theporous film 113 that is obtained becomes too large, and thus the PM 130trapping properties decreases, or the PM 130 passes through the porousfilm 113 and is trapped inside the pore of the partition wall 114, andthus the combustion efficiency is not improved during the filterregeneration treatment. Accordingly, this maximum value is notpreferable. In a case where the filter characteristics are the mainfactor, the upper limit is preferably 7 μm, and is more preferably 5 μm.

Next, in a case where the combustion catalytic operation is the mainfactor, the maximum value of the average primary particle size ispreferably 5 μm. The reason for the limitation is as follows. When theaverage primary particle size exceeds 5 μm, the specific surface area ofthe silicon carbide particle 122 decreases, and thus when the noblemetal particle is carried thereon, the distance between noble metalparticles is reduced. As a result, sintering (grain growth) between thenoble metal particles progresses during use of the exhaust gaspurification filter at a high temperature, and the catalytic activitydecreases, thus this maximum value is not preferable.

The upper limit in a case of setting the combustion catalytic operationas the main factor is preferably 3 μm, and is more preferably 1 μm.

Accordingly, the maximum value of the average primary particle size maybe selected depending on the use or purpose of the DPF 100.

With regard to the silicon carbide particles, examples of a method ofobtaining nanometer-sized particles include a thermal plasma methodusing thermal plasma which has a high temperature and high activity in anon-oxidizing atmosphere and which is easy for introduction to ahigh-speed cooling process. This manufacturing method is useful as amethod of manufacturing silicon carbide nanoparticles which have anaverage primary particle size of approximately 5 nm to 100 nm and whichhave excellent crystallinity. When a raw material with high purity isselected, it is possible to obtain silicon carbide nanoparticles inwhich the amount of impurities is very small.

In addition, a silica precursor sintering method may be exemplified.This method is a method of obtaining silicon carbide particles by bakinga mixture of a material such as an organic silicon compound, siliconsol, and silicic acid hydrogel which contain silicon, a material such asa phenol resin which contains carbon, and a metal compound of lithiumand the like which suppresses grain growth of silicon carbide under anon-oxidizing atmosphere.

In addition, examples of a method of obtaining silicon carbide particleshaving a size distribution from submicron to micron (micrometer) includeindustrial methods such as an Acheson process, a silica reductionmethod, and a silicon carbonization method. In addition, these methodsare already industrially established, and thus a description thereofwill not be repeated here.

The noble metal particles 123, which are carried on the surface of thesilicon carbide particles 121, preferably contain one or two or morekinds of elements selected from a group consisting of platinum (Pt),gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd),osmium (Os), and iridium (Ir).

The average primary particle size of the noble metal particles 123 ispreferably 1 nm to 50 nm, is more preferably 1 nm to 30 nm, and is stillmore preferably 1 nm to 10 nm.

Here, the reason as to why the average primary particle size is limitedto 1 nm to 50 nm is as follows. When the average primary particle sizeis less than 1 nm, the particle size is too small and thus surfaceactivity becomes too strong. Therefore, agglomeration tends to occur,and thus this range is not preferable. On the other hand, when theaverage primary particle size exceeds 50 nm, the noble metal particlesare not coated with the oxide layer that is present on a surface of eachof the silicon carbide particles, and thus the noble metal particlesprotrude from the oxide layer toward the outside and are exposed. As aresult, there is a concern that catalytic characteristics deteriorate,and that the catalytic activity at a low temperature decreases, and thusthis range is not preferable.

The oxide layer 124 is an oxide layer which allows the noble metalparticle 123 to be carried on the surface of the silicon carbideparticles 122 and which is generated on the noble metal particles 123and the silicon carbide particles 122 through oxidation in an oxidizingatmosphere.

The oxide layer 124 has a function of maintaining the noble metalparticles 123 on the surface of the silicon carbide particles 122.Accordingly, the oxide layer 124 suppresses migration of the noble metalparticles 123 under a high-temperature environment, and can prevent adecrease in a surface area due to sintering (grain growth) of the noblemetal particles 123.

It is preferable that the oxide layer 124 be formed from one or twoselected from a group consisting of amorphous SiO, (provided that,0<x≦3) and amorphous SiO_(y)C_(z) (provided that, 0<y≦3 and 0<z≦3). Inaddition, it is not necessary for amorphous SiO, and amorphousSiO_(y)C_(z) to have a single composition, and at each portion of theoxide layer 124, x, y, and z may vary in the above-described range in anarbitrary manner. Further, the oxide layer may further contain one ortwo or more crystalline substances selected from a group consisting ofSiO₂ (silica), SiO, SiOC₃, SiO₂C₂, and SiO₃C. However, the oxide layer124 has a too small thickness and is contained in a trace amount, andthus it is difficult to confirm whether or not a crystalline substanceis contained in the oxide layer 124. In addition, in the followingdescription, substances that form the oxide layer 124 may becollectively described as “oxides of silicon carbide.”

In the oxides of silicon carbide, it is preferable to contain a siliconcarboxide that tends to form a bond with each of the noble metalparticles, in other words, a compound that contains silicon, carbon, andoxygen in combination with each other. When the silicon carboxide formsa bond with the noble metal particles, migration of the noble metalparticles is prevented, and thus the sintering prention effect isfurther improved. In addition, it is known that the melting point of thenoble metal particle 123 becomes lowered from miniaturization. However,when the silicon carboxide forms a bond with the noble metal particle,melting of the noble metal particle 123 can be prevented.

Examples of the silicon carboxide include crystalline SiOC₃, SiO₂C₂, andSiO₃C in addition to amorphous SiO_(y)C_(z) (provided that, 0<y≦3 and0<z≦≦3). In addition, the silicon carboxide may contain silicon, carbon,and oxygen in combination with each other, and may include a compositionother than the above-described composition.

In addition, it is known that the oxides of silicon carbide whichconstitute the oxide layer 124 exhibit oxygen release. It is consideredthat the noble metal particle 123 that is carried promotes an increasein an oxygen release amount in the oxides of silicon carbide andlowering of an oxygen release temperature. Accordingly, even in a lowtemperature region in which a reaction rate of a catalytic reactiondepends on the number of active sites, oxygen released from the oxidesof silicon carbide tends to act as the active site due to an auxiliaryoperation of the noble metal particle. As a result, high combustioncatalytic activity can be obtained, and thus it is considered that PMcombustion properties in a low-temperature region are improved.

[Method of Manufacturing Exhaust Gas Purification Filter]

The method of manufacturing the exhaust gas purification filter of thisembodiment includes a mother material preparing process of preparing thesilicon carbide particles as the mother material, a noble metal particlecarrying process of carrying the noble metal particles on the siliconcarbide particles, a porous film forming process of carrying the siliconcarbide particles on a partition wall which constitutes a filter basebody and is formed from a porous substance to form a porous substance,and an oxide layer forming process of forming an oxide layer on thesurface of the silicon carbide particles.

A sequence of these four processes may be selected in an arbitrarysequence as long as the following conditions are satisfied. That is, themother material preparing process is performed first, and it isnecessary for the oxide layer forming process to be performed withrespect to the silicon carbide particles on which the noble metalparticle is carried (not performed with respect to a silicon carbideparticle alone). In addition, here, the reason as to why it is necessaryfor the oxide layer forming process to be performed with respect to thesilicon carbide particles on which the noble metal particle is carriedis that even when the noble metal particle is carried after forming theoxide layer on the silicon carbide particles, sufficient oxidizingcatalytic characteristics cannot be obtained.

In addition, the noble metal carrying process and the porous filmforming process have similar processes such as dissolution or dispersionin a solvent (dispersion medium), drying and removal of the solvent(dispersion medium), and a heat treatment, and thus both processes maybe simultaneously performed in parallel with each other.

As described above, in the method of manufacturing the exhaust gaspurification filter of this embodiment, the following four processsequences may be selected. In addition, here, the mother materialpreparing process is [A], the noble metal particle carrying process is[B], the porous film forming process is [C], and the oxide layer formingprocess is [D].

In addition, it is assumed that a case of performing a subsequentprocess after an arbitrary process is indicated by “→ (right arrow)”,and a case of simultaneously performing two processes is indicated by“=”.

[A]→[B]→[C]→[D]  (1)

In this method, the noble-metal-particle-carrying silicon carbideparticles are formed, and the porous film is formed using the siliconcarbide particles, and then the surface oxide layer is formed.

[A]→[B]→[D]→[C]  (2)

In this method, after forming the silicon carbide particles on which thenoble metal particle is carried and the surface oxide layer is formed,the porous film is formed using the silicon carbide particles.

[A]→[C]→[B]→[D]  (3)

In this method, after forming the porous film using the silicon carbideparticles as the mother material, the noble metal particles are carriedon the porous film, and then the surface oxide layer is formed.

[A]→[B]=[C]→[D]  (4)

Carrying of the noble metal particle on the silicon carbide particles asthe mother material, and formation of the porous film using the siliconcarbide particles as the mother material are simultaneously performed,and then the surface oxide layer is formed on thenoble-metal-particle-carrying silicon carbide porous film that isobtained.

Next, the respective processes will be described in detail.

“Mother Material Preparing Process”

In this process, the silicon carbide particles as the mother materialare prepared.

The average primary particle size of the silicon carbide particles maybe selected in a range of 0.01 μm to 10 μm in accordance withcharacteristics of the exhaust gas purification filter which aredemanded. When the silicon carbide particles are nanometer-sizedparticles, the silicon carbide particles may be obtained using theabove-described methods, that is, the thermal plasma method, the silicaprecursor baking method, and the like. When the silicon carbideparticles are particles having a size distribution from submicron tomicron (micrometer), the silicon carbide particles may be obtained usingthe Acheson process, the silica reduction method, the siliconcarbonization method, and the like.

“Noble Metal Particle Carrying Process”

In this process, the noble metal particles are carried on the surface ofthe silicon carbide particles as the mother material, or on the surfaceof the silicon carbide particles in the porous film formed from thesilicon carbide particles, thereby forming the noble-metal-carryingsilicon carbide particles or a porous film formed from thenoble-metal-carrying silicon carbide particles.

First, a solution in which noble metal salts as sources of the noblemetal are dissolved, or a dispersion liquid in which noble metalcompound fine particles are dispersed is prepared. As a solvent ordispersion medium, water is preferable. However, in a case where thenoble metal sources are decomposed in water and precipitate, an organicsolvent may be used. As the organic solvent, a polar solvent ispreferable, and alcohols, ketones, and the like are appropriately used.

Next, the silicon carbide particles are immersed and dispersed in thesolution or the dispersion liquid, or the porous film formed from thesilicon carbide particles is immersed in the solution or the dispersionliquid and is dried at a temperature of approximately 60° C. to 250° C.to remove the water or the dispersion medium. According to this, thenoble metal salts or the noble metal compound fine particles may beattached to the surface of the silicon carbide particles (includingparticles forming the porous film).

Next, the porous film or the silicon carbide particles to which thenoble metal salts or the noble metal compound fine particles areattached is subjected to a heat treatment in a reducing atmosphereincluding hydrogen, carbon monoxide, and the like, or in an inertatmosphere such as nitrogen, argon, neon, xenon, and the like, therebyreducing and decomposing the noble metal salts or the noble metalcompound to form the noble metal fine particle. A heat treatmenttemperature or a heat treatment time may be appropriately selecteddepending on the kinds of the noble metal sources, atmosphericconditions, and the like. However, typically, the heat temperature isset to be in a range of 500° C. to 1500° C., and the heat treatment timeis set to be in a range of 10 minutes to 24 hours. In addition, atemperature higher than necessary and time longer than necessary maycause sintering of the silicon carbide particles or sintering of thenoble metal particles that are generated, and thus are not preferable.According to the above-described processes, it is possible to obtain thesilicon carbide particles in which the noble metal fine particles arecarried on a surface, or the porous film formed from the silicon carbideparticles in which the noble metal fine particles are carried on asurface.

Examples of the noble metal sources, which are used in the processes,include salts or compounds which contain one or two or more noble metalelements selected from a group consisting of platinum (Pt), gold (Au),silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os),and iridium (Ir) which are noble metal particles, for example,chlorides, sulfates, nitrates, organic acid salts, complex (complexsalts), hydroxides, and the like.

“Porous Film Forming Process”

The porous film forming process includes a process of dispersing any oneof the silicon carbide particles as the mother material, thenoble-metal-particle-carrying silicon carbide particles, and the siliconcarbide particles on which the noble metal particle is carried and inwhich the surface oxide layer is formed in the dispersion medium toprepare a silicon carbide particle dispersion liquid, and a process ofapplying (coating) the silicon carbide particle dispersion liquid on thepartition wall which constitutes the filter base body and is formed froma porous substance, performing drying, and partially sintering thesilicon carbide particles to form a porous film.

First, any one of the silicon carbide particles, thenoble-metal-particle-carrying silicon carbide particles, and the siliconcarbide particles, on which the noble metal particle is carried and inwhich the surface oxide layer is formed, is dispersed in the dispersionmedium to prepare a silicon carbide particle dispersion liquid.

As the dispersion medium, any dispersion medium may be used as long asthis dispersion medium can uniformly disperse silicon carbide particles,and water or an organic solvent is appropriately used. In addition, anelementary substance of a polymeric monomer or an oligomer or a mixtureof these may be used as necessary.

Examples of the organic solvent that is appropriately used includealcohols such as methanol, ethanol, 1-propanol, 2-propanol, diacetonealcohol, furfuryl alcohol, ethylene glycol, and hexylene glycol; esterssuch as acetic acid methyl ester and acetic acid ethyl ester; etherssuch as diethyl ether, ethylene glycol monomethyl ether (methylcellosolve), ethylene glycol monoethyl ether (ethyl cellosolve),ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, dioxane, andtetrahydrofuran; ketones such as acetone, methyl ethyl ketone,acetylacetone, and acetoacetic ester; acid amides such asN,N-dimethylformamide; aromatic hydrocarbon such as toluene and xylene;and the like. These solvents may be used alone or two or more kindsthereof may be mixed and used.

As the polymeric monomer, an acryl-based monomer or methacryl-basedmonomer such as methyl acrylate and methyl methacrylate, an epoxy-basedmonomer, and the like may be used. In addition, as the oligomer,urethane acrylate-based oligomer, epoxy acrylate-based oligomer,acrylate-based oligomer, and the like may be used.

A dispersing agent (surface treatment agent), a surfactant, apreservative, a stabilizing agent, a defoaming agent, a leveling agent,and the like may be appropriately added to the dispersion liquid tosecure dispersion stability or to improve application property. An addedamount of the dispersing agent, the surfactant, the preservative, thestabilizing agent, the defoaming agent, and the leveling agent is notparticularly limited, and addition may be performed depending on anaddition purpose.

As a device that is applicable to a carrier preparing process, forexample, a kneader, a roll mill, a pin mill, a sand mill, a ball mill,and a planetary ball mill, and the like are exemplary examples, but thesand mill, the ball mill, and the planetary ball mill, and the like arepreferable to disperse the silicon carbide particle in the dispersionmedium using a dispersive medium.

In addition, examples of the dispersive medium such as a ball include aresin-covered body including a metal core formed from steel, lead, andthe like is formed, a sintered body of an inorganic oxide such asalumina, zirconia, silica, and titania, a sintered body of a nitridesuch as silicon nitride, a sintered body of a silicide such as siliconcarbide, glass such as soda glass, lead glass, and high specific gravityglass, but as the dispersive medium that is used in this embodiment,from the viewpoints of mixing and dispersing efficiency, zirconia havinga specific gravity 6 or more, the resin-covered body including the coreformed from steel, and the like are preferable.

Next, the silicon carbide particle dispersion liquid is applied onto thepartition wall formed from the porous substance, and is dried to form acoated film on the partition wall. In addition, the coated film may beformed by immersing the partition wall formed from the porous substancein the silicon carbide particle dispersion liquid, and by pulling up anddrying the partition wall.

As a method of applying the dispersion liquid, it is possible to usetypical wet coating methods such as a bar coat method, a slip castmethod, a wash coat method, and a dip coat method in which thedispersion liquid is applied onto a surface of an object to beprocessed, and the like.

At this time, the viscosity of the dispersion liquid, wetting propertiesof the dispersion liquid on the partition wall, an agglomeration stateof the silicon carbide particles, and the like are controlled in orderfor the dispersion liquid not to enter the inside of the pores in thepartition wall, whereby it is possible to form an independent porousfilm formed from the silicon carbide particles on the partition wall. Inaddition, in a case where the dispersion liquid enters the inside of thepores in the partition wall, a reduction in a pore size or porosity inthe partition wall is caused, and thus a problem of an increase in thepressure loss, and the like occur, and thus this case is not preferable.

In addition, the drying, that is, removal of the water or organicsolvent may be performed in the atmospheric atmosphere at a temperatureof approximately 100° C. to 250° C.

Next, the partition wall in which the coated film of the silicon carbideparticles is formed and which is formed from the porous substance issubjected to a heat treatment under an inert atmosphere such asnitrogen, argon, neon, and xenon, or in a reducing atmosphere such ashydrogen and carbon monoxide at a temperature of 500° C. to 1500° C.,and preferably 600° C. to 1100° C.

According to this heat treatment, the dispersion medium in the coatedfilm, that is, the water or the organic solvent that remains, or thepolymeric monomer or the oligomer in a dried film, or an additive suchas the dispersing agent, the surfactant, the preservative, thestabilizing agent, the defoaming agent, and the leveling agent in thedried film is scattered, and the silicon carbide particles are partiallysintered to form a neck portion in which the particles are bonded toeach other, whereby a porous film bonded in a porous substance shape isformed.

In addition, the drying of the dispersion liquid may be integrated witha heat treatment process as a previous stage of the heat treatmentprocess.

In addition, this heat treatment process may be omitted as long as theoxide layer may be formed on the surface of the silicon carbide particleand silicon carbide particles may be partially sintered by adjustingheat treatment conditions in the oxide layer forming process.

(Case of Simultaneously Performing Noble Metal Particle Carrying Processand Porous Film Forming Process)

The noble metal particle carrying process and the porous film formingprocess are similar processes, and thus both processes may besimultaneously performed in parallel with each other

First, the process of preparing the solution in which the noble metalsalts as the source of the noble metal are dissolved, or the dispersionliquid in which noble metal compound fine particles are dispersed in thenoble metal particle carrying process, and the process of preparing thedispersion medium in the porous film forming process are similar to eachother. In addition, the process of immersing and dispersing the siliconcarbide particle in the solution or the dispersion liquid in the noblemetal particle carrying process and the process of preparing the siliconcarbide particle dispersion liquid in the porous film forming processare similar to each other.

Accordingly, these processes may be collectively performed in a singleprocess by dissolving or dispersing the noble metal sources in thesilicon carbide particle dispersion medium in the porous film formingprocess.

In addition, as the dispersion medium, it is necessary to select asubstance in which the noble metal sources can be easily dissolved ordispersed, and particularly, water may be appropriately used. On theother hand, in a case of using the organic solvent and the like, noblemetal sources that are easily dispersed in the organic solvent may beselected.

Next, the porous film forming process includes a process of applying(coating) the silicon carbide particle dispersion liquid on thepartition wall which constitutes the filter base body and is formed fromthe porous substance, but the noble metal particle carrying process doesnot include a corresponding process.

Next, the process of removing the water or the dispersion medium, whichis a solvent, for drying in the noble metal particle carrying process,and the process of drying the partition wall which is formed from theporous substance and to which the silicon carbide particle dispersionliquid is applied in the porous film forming process are similar to eachother.

These both processes are drying processes and are substantially the sameprocesses, and thus these processes may be performed in a singleprocess. A drying temperature may be set to be in a range that is commonto both processes, that is, a temperature of approximately 100° C. to250° C.

Next, the process of forming the noble metal fine particle by performingthe heat treatment in the reducing atmosphere or the inert atmosphere inthe noble metal particle carrying process, and the process of formingthe porous film by subjecting the partition wall in which the appliedand dried film is formed and which is formed form the porous substanceto the heat treatment in the reducing atmosphere or the inert atmosphereto allow the partition wall to be partially sintered in the porous filmforming process are similar to each other.

Since the both processes are substantially the same processes from theviewpoint of performing the heat treatment in the reducing atmosphere orthe inert atmosphere, these processes may be performed in a singleprocess. A heat treatment temperature or a heat treatment time may beselected in a range common to the both processes in consideration ofnoble metal particle generation conditions or porous film formingconditions.

When performing the above-described processes, that is, the process ofdissolving or dispersing the noble metal sources in the dispersionmedium, the process of dispersing the silicon carbide particle in thedispersion medium, the process of applying the silicon carbide particledispersion liquid that is obtained to the partition wall whichconstitutes the filter base body and is formed from the poroussubstance, the process of drying the coated film that is obtained, andthe process of subjecting the dried and coated film that is obtained tothe heat treatment, the noble metal particle carrying process isintegrated in the porous film forming process. Accordingly, it ispossible to simultaneously perform the noble metal particle carryingprocess and the porous film forming process in parallel with each otheras an integrated process.

“Oxide Layer Forming Process”

In this process, the silicon carbide particles on which the noble metalparticle is carried, or the porous film formed from thenoble-metal-particle-carrying silicon carbide particles is subjected toan oxidizing treatment to form an oxide layer on the surface of thenoble-metal-particle carrying silicon carbide particles.

With regard to the oxidizing treatment, the noble-metal-carrying siliconcarbide particles themselves, or the partition wall, which is formedform the porous substance and which includes the porous film formed fromthe noble-metal-particle-carrying silicon carbide particles, that is, alayer in which noble-metal-carrying silicon carbide particles arepartially sintered, is subjected to the oxidizing treatment in anoxidizing atmosphere such as the air and oxygen at a temperature of 600°C. to 1000° C. and preferably 650° C. to 800° C. for 0.5 hours to 36hours and preferably 4 hours to 12 hours, thereby forming an oxide layeron the surface of the silicon carbide particles.

According to this, it is possible to form the silicon carbide catalystparticle in which the noble metal particle is carried on a surface ofthe silicon carbide particle and the noble metal particle is coated withthe oxide layer, or the porous film formed from the silicon carbideparticle in which the noble metal particle is carried on the surface ofthe silicon carbide particle, and the noble metal particle is coatedwith the oxide layer. That is, when the oxide layer is formed under theabove-described conditions, and the average primary particle size of thenoble metal particle is 50 nm or less, it is possible to form a catalystparticle having high catalytic activity in which the noble metalparticle is completely coated with the oxide layer without beingprotruded from the oxide layer.

According to the above-described processes, it is possible tomanufacture the DPF that is the exhaust gas purification filter of thisembodiment.

As described above, according to the exhaust gas purification filter ofthis embodiment, the silicon carbide particle 122 and the noble metalparticle 123 are contained in the inner wall surface 112 a of thepartition wall 114 formed from the porous substance on the inflow-cell112A side, and the porous film 113, in which the noble metal particle123 is carried on the surface of the silicon carbide particle 121 in astate of being coated with the oxide layer 124, is formed. In addition,the porous film 113 has a pore size smaller than that of the partitionwall 114, and has high porosity and a predetermined film thickness.According to this operation effect, the exhaust gas purification filterof this embodiment has high performance as described below.

First, in the porous film 113, the noble metal particle 123 is presentas a fine particle which is highly dispersed (has large specific surfacearea), and thus the noble metal particle 123 may exhibit catalyticactivity peculiar to the fine particle.

Here, grain growth of the fine noble metal particle progresses from arelatively low temperature in a typical case. However, the noble metalparticle 123 of this embodiment is coated with the oxide layer 124 onthe surface of the silicon carbide particle 122.

The oxide layer 124 is formed from the “oxides of silicon carbide”, andcan form a bond with the noble metal particle, and thus sintering (graingrowth) of the noble metal particle may be prevented. Particularly,since “silicon carboxide” which tends to form a bonding with the noblemetal particle is contained, the effect of preventing the sintering ofthe noble metal particle further increases.

In addition, it is known that in fine noble metal particle 123, themelting point thereof decreases, but melting of the noble metal particle123 may be reduced due to chemical bonding with the oxide layer 124,particularly, SiO_(x)C_(y) silicon carboxide.

In addition, it is known that the oxides of silicon carbide which arecontained in the oxide layer 124 exhibit oxygen release. It isconsidered that the noble metal particle 123 that is carried promotes anincrease in an oxygen release amount in the oxides of silicon carbideand lowering of an oxygen release temperature. Accordingly, even in alow temperature region in which a reaction rate of a catalytic reactiondepends on the number of active sites, it is considered that oxygenreleased from the oxides of silicon carbide tends to act as the activesite due to an auxiliary operation of the noble metal particle.

Further, the silicon carbide is stable at a high temperature, and thusgrain growth is less likely to occur in comparison to oxide particles.According to this, the surface area of the oxides of silicon carbide iseasily maintained, and thus the decrease in the amount of oxygen releasedoes not occur. As a result, even after the exhaust gas purificationfilter of this embodiment is used for a long period of time at a hightemperature, the catalytic activity is easily maintained.

As described above, the exhaust gas purification filter of thisembodiment has high catalytic effect with respect to the PM combustion,and can maintain the combustion catalytic effect for a long period oftime.

Next, the porous film 113 has high PM 130 trapping properties, and themost of PM 130 is trapped on the surface of the porous film 113, anddoes not enter the pores in the porous film 113 or the partition wall114. That is, the most of the PM 130 is trapped by surface layerfiltration, and is not trapped by deep layer filtration. Accordingly, itis possible to prevent the pores from being clogged due to the PM, andthus it is possible to prevent an increase in the pressure loss due tothe clogging while maintaining the PM trapping efficiency.

In addition, due to the presence of the porous film 113 having apredetermined thickness, the flow of the exhaust gas G on the surface ofthe porous film 113 becomes substantially uniform without depending onthe state of the partition wall 114, and thus the PM 130 is uniformlytrapped on the porous film 113. Accordingly, the pressure loss isreduced and thus satisfactory filter characteristics are maintained.

As described above, the exhaust gas purification filter of thisembodiment has high PM trapping properties and low pressure loss, andcan maintain these characteristics.

Next, with regard to a reproducing treatment of combusting and removingthe PM 130 that is trapped and deposited in the exhaust gas purificationfilter of this embodiment, since the PM 130 is directly trapped on theporous film 113 having an combustion catalytic effect and a contact areabetween the PM 130 and the porous film 113 is large, the catalyticeffect can be sufficiently obtained. Further, since the PM 130 isuniformly trapped on the porous film 113, a combustion gas is uniformlysupplied to the PM that is deposited, and thus the PM is uniformlycombusted. According to this, PM combustion and removal may be performedat a low temperature and in a short time.

Particularly, when the catalytic effect is high, the PM combustion canbe performed at a temperature of the exhaust gas during a typicaloperation of a diesel engine, and thus special operation for theregeneration treatment is not necessary, and thus continuous PMcombustion can be performed.

Further, the PM 130, which is trapped in the pores of the porous film113 or the partition wall 114, causes rapid combustion, therebypartially causing rapid temperature rising in the exhaust gaspurification filter, and thus deterioration or breakage of the exhaustgas purification filter is caused. However, in the exhaust gaspurification filter of this embodiment, the PM 130 is trapped by thesurface filtration, and is not present in the pores of the porous film113 or the partition wall 114, and thus the above-described problem doesnot occur.

As described above, the exhaust gas purification filter of thisembodiment can prevent an increase in the pressure loss whilemaintaining high PM trapping properties. In addition, lowering of a PMcombustion temperature or shortening of a PM combustion time during thefilter regeneration may be realized, and thus durability of the filteris improved. In addition, continuous regeneration is also possible.

Accordingly, in a vehicle that is provided with the exhaust gaspurification filter of this embodiment, it is possible to continuouslycombust the PM while maintaining high PM trapping properties and lowpressure loss, and maintaining PM combustion efficiency. As a result, itis possible to provide a vehicle which is excellent in exhaust gaspurification properties and is also excellent in driving performancewith low power consumption and low cost.

EXAMPLES

Hereinafter, the invention will be described in detail with reference toExamples and Comparative Examples, but the invention is not limited tothese Examples. In addition, as a honeycomb-structured base materialformed from silicon carbide in the following Examples and ComparativeExamples, a base material having an average pore size of 12 μm andaverage porosity of 45% at the partition wall was used.

[Each Measurement and Evaluation Method]

First, a description will be made with respect to an evaluation methodin the exhaust gas purification filters of Examples and ComparativeExamples.

(1) Composition of Oxide Layer

The porous film in the filter base body was cut for each honeycomb basematerial and was measured by using an X-ray photoelectric analyzer(XPS/ESCA) (Sigma Probe, manufactured by VG-Scientific Co.) to performsurface composition analysis with respect to a surface of a SiC particlethat forms the porous film.

(2) Carried State of Noble Metal Particle and Average Primary ParticleSize

The porous film in the filter base body was cut for each honeycomb basematerial, and was observed by using a field emission transmissionelectron microscope (FE-TEM) (JEM-2100F, manufactured by JapanElectronic Co., Ltd) to evaluate a carried state of the noble metalparticle.

In addition, with regard to the average primary particle size of thenoble metal particle, the primary particle sizes of 500 noble metalparticles, which were randomly selected from an observed image obtainedin the same manner, were measured, and the average value thereof was setto the average primary particle size of the noble metal particle.

(3) Pore Size and Average Porosity of Porous Film

A pore size distribution at a porous film portion in the filter basebody was measured using a mercury porosimeter (Pore Master 60GT,manufactured by Quantachrome Co., Ltd.), and 50% accumulation of mercuryentrance volume was used as the average pore size of the porous film. Inaddition, the average porosity of the porous film was measured by thesame mercury porosimeter.

(4) PM Combustion Test

(A) Preparation of Evaluation Sample

A test specimen having dimensions of 4.5 mm×4.5 mm×7 mm was cut from thefilter base body by using a diamond cutter. The test specimen has ahoneycomb body cell opening (in a cross-sectional direction of a gasflow passage) of 4.5 mm×4.5 mm, and a honeycomb body cell length (gasflow passage length) of 7 mm, and the test specimen is configured bynine (3×3) gas flow passages. In addition, in the nine gas flowpassages, the porous film was not present in a total of five gaspassages on the center and on four corners, and the porous film wasformed on a surface of the partition wall that constitutes the remainingfour gas flow passages.

Next, among the gas flow passages of the test specimen, with respect tothe gas flow passage in which the porous film was formed on the surfaceof the partition wall, an exhaust gas outlet side was covered with aheat-resistant sealing material, and with respect to a gas flow passagein which the porous film was not formed, an exhaust gas inlet side wascovered with the heat-resistant sealing material to obtain the shape ofthe exhaust gas purification filter, and then this test specimen wasused an evaluation sample.

(B) PM Trapping

As a PM generation source, a lamp in which a diesel oil was filled wasused. After stabilizing a flame, an exhaust gas containing PM was suckedfrom a base portion of a diffused flame, and this exhaust gas wasallowed to flow through the evaluation sample, and the PM was depositedon the evaluation sample.

(C) PM Combustion Test

A high-temperature mixed gas of oxygen and nitrogen was allowed to flowto the evaluation sample on which the PM was deposited, and atemperature of the evaluation sample was increased to combust the PM inthe evaluation sample.

Each flow rate of oxygen and nitrogen in the mixed gas was controlledusing amass flow meter. The flow rate of oxygen was set to be in a rangeof 200 ml/minute, and the flow rate of nitrogen was set to be in a rangeof 2800 ml/minute (at room temperature). The mixed gas was heated to775° C. to 850° C., and was allowed to flow through the evaluationsample.

With respect to gas after passing through the evaluation sample, anamount of CO and an amount of CO₂ were continuously measured by aninfrared-type gas concentration measuring device to perform gas analysisalong with the PM combustion. In addition, a thermocouple was insertedin the gas flow passage of the evaluation sample on a gas discharge sideto measure a temperature of the evaluation sample.

From these results, the evaluation sample temperature at which theconcentration of CO and CO₂ is maximized was used as the PM combustiontemperature.

(5) Oxygen Desorption Temperature

To confirm the oxygen adsorption and desorption behavior in the siliconcarbide particle which constitutes the porous film of the exhaust gaspurification filters of Examples and Comparative Examples, on which thenoble metal particle is carried and the oxide layer is formed, an oxygendesorption temperature was measured. Here, measurement using thefollowing two methods was performed with respect to respective samplesmanufactured in the same manner as Examples and Comparative Examplesexcept that the process of forming the porous film on thehoneycomb-structured base material (formation of an application liquidand formation of a coated film on the base material) was omitted so asto obtaine a powder sample.

(A) Hydrogen Temperature Programmed Reduction (H₂-TPR)

A temperature of oxygen release from silicon carbide particle wasmeasured using a catalyst evaluation device (Bell CAT B, manufactured byJapan BEL Co., Ltd.). First, a sample was subjected to a pre-treatmentin a mixed gas of oxygen (5% by volume) and helium (remainder) at 800°C. for one hour to allow oxygen to be adsorbed to the silicon carbideparticles. Then, the temperature was lowered to room temperature.Subsequently, the temperature of the sample was increased to 800° C. ina mixed gas of hydrogen (5% by volume) and argon (remainder) at a rateof 10° C./minute, and a generated gas was analyzed using a thermalconductivity detector to measure hydrogen consumption (oxygen release)temperature in the silicon carbide particles.

(B) Vacuum Thermal Desorption Spectrometry (TDS)

As a method of confirming the oxygen desorption behavior of the siliconcarbide particles in vacuum, thermal desorption spectrometry wasperformed. First, a sample was subjected to a pre-treatment in a mixedgas of oxygen (10% by volume) and nitrogen (remainder) for one hour toallow oxygen to be adsorbed to the silicon carbide particles. Here, anadsorption temperature was set to two kinds of 25° C. and 600° C. Then,the temperature of the sample which was subjected to the pre-treatmentat 600° C. was lowered to room temperature. Subsequently, each samplewas disposed in a measurement container, and a temperature was increasedin vacuum to 1100° C. at a rate of 40° C./minute, the amount ofgenerated oxygen was measured using a four pile polar mass spectrometerto confirm temperature dependency of the oxygen desorption behavior.

Example 6

15 g of silicon carbide particles having an average primary particlesize of 0.035 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.01 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 10.0%by volume, 87.5% by volume, and 2.5% by volume. Next, theplatinum-salt-carrying silicon carbide particles and water were mixedwith a ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 12 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Example 6.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the platinum-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=980° C., retention time=80 minutes, andatmosphere=argon

Second step: temperature=730° C., retention time=360 minutes, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a filter base body ofExample 6.

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of3 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles, and theplatinum fine particle was coated with the oxide layer.

In addition, the average pore size of the porous film was 0.1 μm, andthe average porosity was 75%.

The oxygen desorption temperature by H₂-TPR was 335° C., and the PMcombustion temperature was 350° C.

These results are collectively shown in Table 3.

FIG. 10 is a scanning electron microscope image (SEM image) of theporous film of Example 6, FIG. 11 is an FE-TEM image of the porous film,and FIG. 12 is an explanatory view illustrating a structure of theporous film of FIG. 11.

According to the drawings, it can be seen that in thenoble-metal-carrying silicon carbide particles, the noble metal particle(platinum particle) was carried on the surface of the silicon carbideparticles, and the noble metal particle (platinum particle) was carriedon the surface of the silicon carbide particles in a state of beingcoated with the oxide layer (oxide film). In addition, in the FE-TEMimage, a crystal lattice image was not recognized in the oxide layer(oxide film), and thus it was discovered that the oxide layer (oxidefilm) was an amorphous substance.

Example 7

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of silver nitrate was added to the resultant slurry,which was obtained, in such a manner that silver became 0.1 g on thebasis of 1 g of silicon carbide particles, and a dispersion treatmentusing zirconia beads as the dispersive medium was performed again for 30minutes. Then, evaporation and drying were performed to preparesilver-salt-carrying silicon carbide particles.

Next, the silver-salt-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 10.0%by volume, 87.5% by volume, and 2.5% by volume. Next, thesilver-salt-carrying silicon carbide particles and water were mixed witha ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 12 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained, and mixingwas performed for 20 minutes, thereby obtaining a silicon carbideparticle dispersion liquid (application liquid) of Example 7.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the silver-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=500° C., retention time=60 minutes, andatmosphere=nitrogen

Second step: temperature=600° C., retention time=36 hours, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, formation of silver (noblemetal) fine particles by reduction and decomposition of silver nitratecarried on the surface of the silicon carbide particles, and formationof a surface oxide layer of the silicon carbide particles wereperformed, thereby preparing a filter base body of Example 7.

In the filter base body that was obtained, a porous film formed fromsilicon carbide particles was formed on the honeycomb base material, andthe silver fine particle having an average primary particle size of 50nm was carried on the surface of the silicon carbide particles. Inaddition, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles, and thesilver fine particle was coated with the oxide layer. In addition, as isthe case with Example 6, from the FE-TEM image, it was discovered thatthe oxide layer was an amorphous substance.

In addition, the average pore size of the porous film was 0.30 μm, andthe average porosity was 71%.

The oxygen desorption temperature by H₂-TPR was 335° C., and the PMcombustion temperature was 300° C.

These results are collectively shown in Table 3.

Example 8

15 g of silicon carbide particles having an average primary particlesize of 0.015 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.05 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles weresubjected to a heat treatment under the following conditions. Inaddition, a first step corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=600° C., retention time=90 minutes, andatmosphere=argon Third step: temperature=700° C., retention time=60minutes, and atmosphere=air

Through this heat treatment, formation of platinum (noble metal) fineparticles by reduction and decomposition of dinitro-platinate carried onthe surface of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebypreparing noble-metal-carrying silicon carbide particles in which theoxide layer was formed.

Next, the noble-metal-carrying silicon carbide particles in which theoxide layer was formed, water, and gelatin used as a gelating agent wereweighed in content ratios of 6.0% by volume, 93.0% by volume, and 1.0%by volume. Next, the noble-metal-carrying silicon carbide particles inwhich the oxide layer was formed and water were mixed with a ball millusing a resin ball including an iron core at a rotation speed of 220 rpmfor 48 hours to obtain a dispersion liquid. Then, the gelatin was addedto the dispersion liquid that was obtained and mixing was performed for20 minutes to obtain a silicon carbide particle dispersion liquid(application liquid) of Example 8.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film, which was formed from thenoble-metal-carrying silicon carbide particles in which the oxide layerwas formed, on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Second step: temperature=700° C., retention time=12 hours, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebypreparing a filter base body of Example 8.

In the filter base body that was obtained, a porous film formed from thesilicon carbide particles was formed on the honeycomb base material, anda platinum fine particle having an average primary particle size of 10nm was carried on the surface of the silicon carbide particles. Further,an oxide layer, which was formed from an amorphous compound (SiO_(x),provided that, 0<x≦3) containing silicon and oxygen in combination andan amorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the platinum fine particlewas coated with the oxide layer. In addition, as is the case withExample 6, from the FE-TEM image, it was discovered that the oxide layerwas an amorphous substance.

In addition, the average pore size of the porous film was 0.08 μm, andthe average porosity was 89%.

In addition, the PM combustion temperature was 450° C.

These results are collectively shown in Table 3.

Example 9

15 g of silicon carbide particles having an average primary particlesize of 0.04 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of silver nitrate was added to the resultant slurry,which was obtained, in such a manner that silver became 0.01 g on thebasis of 1 g of silicon carbide particles, and a dispersion treatmentusing zirconia beads as the dispersive medium was performed again for 30minutes. Then, evaporation and drying were performed to preparesilver-salt-carrying silicon carbide particles.

Next, the silver-salt-carrying silicon carbide particles were subjectedto a heat treatment under the following conditions. In addition, a firststep corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=600° C., retention time=90 minutes, andatmosphere=argon

Third step: temperature=700° C., retention time=60 minutes, andatmosphere=air

Through this heat treatment, formation of silver (noble metal) fineparticles by reduction and decomposition of silver nitrate carried onthe surface of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebypreparing noble-metal-carrying silicon carbide particles in which theoxide layer was formed.

Next, the noble-metal-carrying silicon carbide particles in which theoxide layer was formed, water, and gelatin used as a gelating agent wereweighed in content ratios of 22% by volume, 75.5% by volume, and 2.5% byvolume. Next, the noble-metal-carrying silicon carbide particles inwhich the oxide layer was formed and water were mixed with a ball millusing a resin ball including an iron core at a rotation speed of 220 rpmfor 48 hours to obtain a dispersion liquid. Then, the gelatin was addedto the dispersion liquid that was obtained and mixing was performed for20 minutes to obtain a silicon carbide particle dispersion liquid(application liquid) of Example 9.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film, which was formed from thenoble-metal-carrying silicon carbide particles in which the oxide layerwas formed, on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=900° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=600° C., retention time=24 hours, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, and formation of a surfaceoxide layer of the silicon carbide particles were performed, therebypreparing a filter base body of Example 9.

In the filter base body that was obtained, a porous film formed from thesilicon carbide particles was formed on the honeycomb base material, anda silver fine particle having an average primary particle size of 30 nmwas carried on the surface of the silicon carbide particles. Further, anoxide layer, which was formed from an amorphous compound (SiO_(x),provided that, 0<x≦3) containing silicon and oxygen in combination andan amorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the silver fine particlewas coated with the oxide layer. In addition, as is the case withExample 6, from the FE-TEM image, it was discovered that the oxide layerwas an amorphous substance.

In addition, the average pore size of the porous film was 0.2 μm, andthe average porosity was 62%.

In addition, the PM combustion temperature was 410° C.

These results are collectively shown in Table 3.

Example 10

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of 0.8μm were added to the slurry that was obtained, thereby preparing a mixedliquid in which an amount of platinum was adjusted to be 0.01 g on thebasis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 20.5% byvolume, the content ratio of water became 77.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 10. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 10.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film, which was formed from theplatinum-salt-carrying silicon carbide particles, on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=900° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=600° C., retention time=480 minutes, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a filter base body ofExample 10.

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of5 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles, and theplatinum fine particle was coated with the oxide layer. In addition, asis the case with Example 6, from the FE-TEM image, it was discoveredthat the oxide layer was an amorphous substance.

In addition, the average pore size of the porous film was 1.0 μm, andthe average porosity was 68%.

The oxygen desorption temperature by TDS was 721° C., and the PMcombustion temperature was 430° C.

These results are collectively shown in Table 3.

Example 11

15 g of silicon carbide particles having an average primary particlesize of 0.06 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of palladium nitrate was added to the resultantslurry, which was obtained, in such a manner that palladium became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of 5.0μm were added to the slurry that was obtained, thereby preparing a mixedliquid in which an amount of palladium was adjusted to be 0.01 g on thebasis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 35.0% byvolume, the content ratio of water became 63.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 1.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 11. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 11.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the palladium-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=850° C., retention time=240 minutes, andatmosphere=argon

Second step: temperature=800° C., retention time=360 minutes, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, formation of palladium(noble metal) fine particles by reduction and decomposition of palladiumnitrate carried on the surface of the silicon carbide particles, andformation of a surface oxide layer of the silicon carbide particles wereperformed, thereby preparing a filter base body of Example 11.

In the filter base body that was obtained, a porous film formed from thesilicon carbide particles was formed on the honeycomb base material, anda palladium fine particle having an average primary particle size of 20nm was carried on the surface of the silicon carbide particles. Further,an oxide layer, which was formed from an amorphous compound (SiO_(x),provided that, 0<x≦3) containing silicon and oxygen in combination andan amorphous compound (SiO_(y)C_(z), provided that, 0<y≦3 and 0<z≦3)containing silicon, oxygen, and carbon in combination, was formed on thesurface of the silicon carbide particles, and the palladium fineparticle was coated with the oxide layer. In addition, as is the casewith Example 6, from the FE-TEM image, it was discovered that the oxidelayer was an amorphous substance.

In addition, the average pore size of the porous film was 2.0 μm, andthe average porosity was 55%.

The oxygen desorption temperature by H₂-TPR was 498° C., and the PMcombustion temperature was 470° C.

These results are collectively shown in Table 3.

Example 12

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining slurry.

Silicon carbide particles having an average primary particle size of10.0 μm were added to the slurry that was obtained, thereby preparing amixed liquid in which an amount of platinum was adjusted to be 0.005 gon the basis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 30.0% byvolume, the content ratio of water became 67.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofExample 12. First, ammonium polycarboxylate as a surfactant was added tothe mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of Example 12.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the platinum-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=1000° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=730° C., retention time=60 minutes, andatmosphere=air

Through this heat treatment, formation of a porous film by partialsintering of the silicon carbide particles, formation of platinum (noblemetal) fine particles by reduction and decomposition ofdinitro-platinate carried on the surface of the silicon carbideparticles, and formation of a surface oxide layer of the silicon carbideparticles were performed, thereby preparing a filter base body ofExample 12.

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of1 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles, and theplatinum fine particle was coated with the oxide layer. In addition, asis the case with Example 6, from the FE-TEM image, it was discoveredthat the oxide layer was an amorphous substance.

In addition, the average pore size of the porous film was 3.0 μm, andthe average porosity was 51%.

In addition, the PM combustion temperature was 500° C.

These results are collectively shown in Table 3.

Comparative Example 4

15 g of silicon carbide particles having an average primary particlesize of 0.035 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.01 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 10.0%by volume, 87.5% by volume, and 2.5% by volume. Next, theplatinum-salt-carrying silicon carbide particles and water were mixedwith a ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 12 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Comparative Example 4.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the platinum-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions, therebypreparing a filter base body of Comparative Example 4.

First step: temperature=980° C., retention time=70 minutes, andatmosphere=argon

Second step: temperature=450° C., retention time=30 minutes, andatmosphere=air

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of1 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer was not formed on the surface of the siliconcarbide particles, and thus the platinum fine particle was not alsocoated with the oxide layer.

In addition, the average pore size of the porous film was 0.05 μm, andthe average porosity was 73%.

In addition, the PM combustion temperature was 590° C.

These results are collectively shown in Table 3.

Comparative Example 5

15 g of silicon carbide particles having an average primary particlesize of 0.04 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.05 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes. Then, evaporation and drying were performed toprepare platinum-salt-carrying silicon carbide particles.

Next, the platinum-salt-carrying silicon carbide particles weresubjected to a heat treatment under the following conditions. Throughthe heat treatment, formation of platinum (noble metal) fine particlesby reduction and decomposition of dinitro-platinate carried on thesurface of the silicon carbide particles was performed, thereby formingthe noble-metal-carrying silicon carbide particles. In addition, a firststep corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Third step: temperature=450° C., retention time=240 minutes, andatmosphere=air

Next, the noble-metal-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 23.0%by volume, 76.0% by volume, and 1.0% by volume. Next, thenoble-metal-carrying silicon carbide particles and water were mixed witha ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 48 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Comparative Example 5.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the noble-metal-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions, therebypreparing a filter base body of Comparative Example 5.

First step: temperature=1000° C., retention time=30 minutes, andatmosphere=argon

Second step: temperature=550° C., retention time=120 minutes, andatmosphere=air

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of1.5 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer was not formed on the surface of the siliconcarbide particles, and thus the platinum fine particle was not alsocoated with the oxide layer.

In addition, the average pore size of the porous film was 0.04 μm, andthe average porosity was 68%.

In addition, the PM combustion temperature was 600° C.

These results are collectively shown in Table 3.

Comparative Example 6

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of dinitro-platinate was added to the resultantslurry, which was obtained, in such a manner that platinum became 0.1 gon the basis of 1 g of silicon carbide particles, and a dispersiontreatment using zirconia beads as the dispersive medium was performedagain for 30 minutes, thereby obtaining a slurry.

Silicon carbide particles having an average primary particle size of10.0 μm were added to the slurry that was obtained, thereby preparing amixed liquid in which an amount of platinum was adjusted to be 0.005 gon the basis of 1 g of the silicon carbide particles in the slurry.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 30.0% byvolume, the content ratio of water became 67.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofComparative Example 6. First, ammonium polycarboxylate as a surfactantwas added to the mixed liquid, water in an amount needed to realize theabove-described ratio was added to the mixed liquid, and then theresultant mixture was mixed with a ball mill using a resin ballincluding an iron core at a rotation speed of 220 rpm for 48 hours toobtain a dispersion liquid. Next, gelatin was added to the dispersionliquid in an amount needed to realize the above-described ratio, andthen mixing was performed for 20 minutes, thereby obtaining a siliconcarbide particle dispersion liquid (application liquid) of ComparativeExample 6.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the platinum-salt-carryingsilicon carbide particles on the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions, therebypreparing a filter base body of Comparative Example 6.

First step: temperature=1000° C., retention time=360 minutes, andatmosphere=argon

Second step: temperature=500° C., retention time=360 minutes, andatmosphere=air

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a platinum fine particle having an average primary particle size of3 nm was carried on the surface of the silicon carbide particles.Further, an oxide layer was not formed on the surface of the siliconcarbide particles, and thus the platinum fine particle was not alsocoated with the oxide layer.

In addition, the average pore size of the porous film was 3.0 μm, andthe average porosity was 51%.

In addition, the PM combustion temperature was 620° C.

These results are collectively shown in Table 3.

Comparative Example 7

15 g of silicon carbide particles having an average primary particlesize of 0.04 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

An aqueous solution of silver nitrate was added to the resultant slurry,which was obtained, in such a manner that silver became 0.01 g on thebasis of 1 g of silicon carbide particles, and a dispersion treatmentusing zirconia beads as the dispersive medium was performed again for 30minutes. Then, evaporation and drying were performed to preparesilver-salt-carrying silicon carbide particles.

Next, the silver-salt-carrying silicon carbide particles were subjectedto a heat treatment under the following conditions, and formation ofsilver (noble metal) fine particle by reduction and decomposition ofsilver nitrate carried on the surface of the silicon carbide particleswas carried out, thereby forming noble-metal-carrying silicon carbideparticles. In addition, a first step corresponds to a drying process.

First step: temperature=120° C., retention time=24 hours, andatmosphere=air

Second step: temperature=650° C., retention time=90 minutes, andatmosphere=argon

Third step: temperature=450° C., retention time=60 minutes, andatmosphere=air

Next, the noble-metal-carrying silicon carbide particles, water, andgelatin used as a gelating agent were weighed in content ratios of 31.5%by volume, 66.0% by volume, and 2.5% by volume. Next, thenoble-metal-carrying silicon carbide particles and water were mixed witha ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 48 hours to obtain a dispersion liquid. Then, thegelatin was added to the dispersion liquid that was obtained and mixingwas performed for 20 minutes to obtain a silicon carbide particledispersion liquid (application liquid) of Comparative Example 7.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming an applied and dried film formed from thenoble-metal-carrying silicon carbide particles on the base material.

Next, the base material on which the applied and dried film was formedwas subjected to a heat treatment under the following conditions,thereby preparing a filter base body of Comparative Example 7.

First step: temperature=900° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=520° C., retention time=90 minutes, andatmosphere=air

In the filter base body that was obtained, the porous film formed fromthe silicon carbide particles was formed on the honeycomb base material,and a silver fine particle having an average primary particle size of 10nm was carried on the surface of the silicon carbide particles. Further,an oxide layer was not formed on the surface of the silicon carbideparticles, and thus the silver fine particle was not also coated withthe oxide layer.

In addition, the average pore size of the porous film was 0.5 μm, andthe average porosity was 58%.

In addition, the PM combustion temperature was 570° C.

These results are collectively shown in Table 3.

Comparative Example 8

15 g of silicon carbide particles having an average primary particlesize of 0.035 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

The slurry that was obtained was evaporated and dried, thereby preparinga surface-treated silicon carbide particles.

Next, the surface-treated silicon carbide particles, water, and gelatinused as a gelating agent were weighed in content ratios of 10.0% byvolume, 87.5% by volume, and 2.5% by volume. Next, the surface-treatedsilicon carbide particles and water were mixed with a ball mill using aresin ball including an iron core at a rotation speed of 220 rpm for 12hours to obtain a dispersion liquid. Then, the gelatin was added to thedispersion liquid that was obtained and mixing was performed for 20minutes to obtain a silicon carbide particle dispersion liquid(application liquid) of Comparative Example 8.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the silicon carbide particleson the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=980° C., retention time=80 minutes, andatmosphere=argon

Second step: temperature=730° C., retention time=360 minutes, andatmosphere=air

Through this heat treatment, a filter base body of Comparative Example8, in which a porous film was formed by partial sintering of the siliconcarbide particles and an oxide layer was formed on the surface of thesilicon carbide particles, was prepared.

In the filter base body that was obtained, a porous film formed from thesilicon carbide particles was formed on the honeycomb base material.Further, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles. In addition,as is the case with Example 6, from the FE-TEM image, it was discoveredthat the oxide layer was an amorphous substance.

In addition, the average pore size of the porous film was 0.1 μm, andthe average porosity was 75%.

The oxygen desorption temperature by H₂-TPR was 453° C., and the PMcombustion temperature was 500° C.

These results are collectively shown in Table 3.

Comparative Example 9

15 g of silicon carbide particles having an average primary particlesize of 0.030 μm was added to a dispersion medium in which ammoniumpolycarboxylate as a surfactant and a deforming agent were dissolved in80 g of pure water, and in this state, a dispersion treatment usingzirconia beads as a dispersive medium was performed for 180 minutes.

135 g of silicon carbide particles having an average primary particlesize of 0.8 μm was added to the slurry that was obtained, and mixing wasperformed, thereby preparing a mixed liquid.

Water and gelatin were added to the mixed liquid in such a manner thatthe content ratio of the silicon carbide particles became 20.5% byvolume, the content ratio of water became 77.5% by volume, and thecontent ratio of gelatin used as a gelating agent became 2.5% by volume,thereby adjusting a silicon carbide particle dispersion liquid ofComparative Example 9.

First, ammonium polycarboxylate as a surfactant was added to the mixedliquid, water in an amount needed to realize the above-described ratiowas added to the mixed liquid, and then the resultant mixture was mixedwith a ball mill using a resin ball including an iron core at a rotationspeed of 220 rpm for 48 hours to obtain a dispersion liquid. Next,gelatin was added to the dispersion liquid in an amount needed torealize the above-described ratio, and then mixing was performed for 20minutes, thereby obtaining a silicon carbide particle dispersion liquid(application liquid) of Comparative Example 9.

Next, a honeycomb-structured base material formed from silicon carbidewas immersed in the silicon carbide particle dispersion liquid. Then,the base material was pulled up, and was dried at 100° C. for 12 hours,thereby forming a coated film formed from the silicon carbide particleson the base material.

Next, the base material on which the coated film was formed wassubjected to a heat treatment under the following conditions.

First step: temperature=900° C., retention time=120 minutes, andatmosphere=argon

Second step: temperature=600° C., retention time=480 minutes, andatmosphere=air

Through this heat treatment, a filter base body of Comparative Example9, in which a porous film was formed by partial sintering of the siliconcarbide particles and an oxide layer was formed on the surface of thesilicon carbide particles, was prepared.

In the filter base body that was obtained, a porous film formed from thesilicon carbide particles was formed on the honeycomb base material.Further, an oxide layer, which was formed from an amorphous compound(SiO_(x), provided that, 0<x≦3) containing silicon and oxygen incombination and an amorphous compound (SiO_(y)C_(z), provided that,0<y≦3 and 0<z≦3) containing silicon, oxygen, and carbon in combination,was formed on the surface of the silicon carbide particles. In addition,as is the case with Example 6, from the FE-TEM image, it was discoveredthat the oxide layer was an amorphous substance.

In addition, the average pore size of the porous film was 0.1 μm, andthe average porosity was 68%.

The oxygen desorption temperature by TDS was 735° C., and the PMcombustion temperature was 550° C.

These results are collectively shown in Table 3.

TABLE 3 Average primary Particle particle size Average Average Oxygendesorption PM combustion Noble size of of noble pore size porositytemperature (° C.) temperature metal Oxide layer SiC (μm) metal (nm)(μm) (%) H2-TPR TDS (° C.) Example 6 Pt SiO_(x) (0 < x ≦ 3), 0.035 3 0.175 335 — 350 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Example 7 Ag SiO_(x) (0< x ≦ 3), 0.03 50 0.3 71 435 — 300 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3)Example 8 Pt SiO_(x) (0 < x ≦ 3), 0.015 10 0.08 89 — — 450 SiO_(y)C_(z)(0 < y ≦ 3, 0 < z ≦ 3) Example 9 Ag SiO_(x) (0 < x ≦ 3), 0.04 30 0.2 62— — 410 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Example 10 Pt SiO_(x) (0 < x≦ 3), 0.030 5 1 68 — 721 430 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) 0.08Example 11 Pd SiO_(x) (0 < x ≦ 3), 0.06 20 2 55 498 — 470 SiO_(y)C_(z)(0 < y ≦ 3, 0 < z ≦ 3) 5.0 Example 12 Pt SiO_(x) (0 < x ≦ 3), 0.030 1 351 — — 500 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) 10 Comparative Pt Oxidefilm is not formed 0.035 1 0.05 73 — — 590 Example 13 Comparative PtOxide film is not formed 0.04 1.5 0.04 68 — — 600 Example 4 ComparativePt Oxide film is not formed 0.030 3 3 51 — — 620 Example 5 10Comparative Ag Oxide film is not formed 0.04 10 0.5 58 — — 570 Example 6Comparative Without SiO_(x) (0 < x ≦ 3), 0.035 — 0.1 75 453 — 500Example 7 SiO_(y)C_(z) (0 < y ≦ 3, 0 < z ≦ 3) Comparative WithoutSiO_(x) (0 < x ≦ 3), 0.030 — 1 68 — 735 550 Example 8 SiO_(y)C_(z) (0 <y ≦ 3, 0 < z ≦ 3) 0.08

As could be seen from the above-described results, in the filter basebodies of Examples 6 to 12, a nanometer-sized noble metal fine particlewas carried on the surface of the silicon carbide particles, the oxidelayer was formed on the surface of the silicon carbide particles, andthe noble metal fine particles were coated with the oxide layer.

The PM combustion temperature in Examples 6 to 12 was as low as 300° C.to 500° C., and a sufficient catalytic effect with respect to PMcombustion was obtained. In addition, the oxygen desorption temperatureby H₂-TPR was also close to the value of the PM combustion temperature,and thus the catalytic effect with respect to the PM combustion wasproved. In addition, in Example 7, the oxygen desorption temperature byH₂-TPR was higher in comparison to the PM combustion temperature, butthis is considered to be because of an oxygen donating properties ofsilver with respect to PM are higher in comparison to hydrogen.

On the other hand, in the filter base bodies of Comparative Examples 4to 7, as is the case with Examples, the nanometer-sized noble metal fineparticle were carried on the surface of the silicon carbide particles.However, the oxide layer was not formed on the surface of the siliconcarbide particles, and thus the noble metal fine particle carried on thesurface of the silicon carbide particles were in an exposed statewithout being coated with the oxide layer.

In Comparative Examples 4 to 7, the PM combustion temperature was 570°C. to 620° C., and was higher by approximately 100° C. to 300° C. incomparison to Examples, and thus it was proved that a sufficientcatalytic effect with respect to the PM combustion was not obtained.

In addition, in the filter base bodies of Comparative Examples 8 and 9,the oxide layer was formed on the surface of the silicon carbideparticles, but the noble metal fine particles were not carried.

In the filter base bodies of Comparative Examples 8 and 9, the PMcombustion temperature was 500° C. to 550° C. and was lower incomparison to Comparative Examples 4 to 7. However, the PM combustiontemperature was higher by 120° C. to 150° C. in comparison to Examples 6and 10, which contained the same silicon carbide particles and containedplatinum as a noble metal, and the catalytic effect with respect to thePM combustion was not sufficient.

Further, in Example 10 and Comparative Example 9, the oxygen desorptiontemperature by TDS was higher in comparison to the PM combustiontemperature or the value of the oxygen desorption temperature by H₂-TPR.The reason for the result is considered to be as follows. At the oxygendesorption temperature by TDS, oxygen is only desorbed in the vacuum,but at the oxygen desorption temperature by PM combustion or H₂-TPR,oxygen acceptors (materials to be oxidized) such as PM (carbon) andhydrogen is present, and these materials react with desorbed oxygen. Asa result, the desorption of oxygen is simplified.

INDUSTRIAL APPLICABILITY

The invention is applicable to an exhaust purification catalyst thatpurifies an exhaust gas discharged from an internal combustion engine,and an exhaust emission control device for an internal combustion enginein which the exhaust purification catalyst is disposed in an exhaustpassage of the internal combustion engine, and more particularly, to anexhaust purification catalyst that efficiently purifies carbon monoxide(CO), hydrocarbon (HC), nitrogen oxide (NOx), particulate matter (PM),and the like which are contained in an exhaust gas discharged from theinternal combustion engine of an engine and the like, and an exhaustemission control device for the internal combustion engine in which theexhaust purification catalyst is disposed in the exhaust passage.

In addition, the invention is also applicable to an exhaust gaspurification filter that is very suitable for removing the particulatematter from the exhaust gas discharged from an diesel engine of avehicle and the like, and more particularly, to an exhaust gaspurification filter which prevents an increase in pressure loss whileimproving particulate matter trapping properties, and which lowers acombustion temperature of the particulate matter during regeneration ofthe filter, thereby realizing improvement in durability and continuousregeneration of the filter.

REFERENCE SIGNS LIST

-   -   1: Noble-metal-carrying silicon carbide particle    -   2: Silicon carbide particle    -   3: Noble metal particle    -   4: oxide layer    -   11: Exhaust emission control device    -   12: Tubular exhaust passage    -   13: Honeycomb base material    -   14: Bottle    -   15: Heating furnace    -   20: Porous film    -   21 a, 21 b: Noble-metal-carrying silicon carbide particle    -   22: Silicon carbide particle    -   23: Noble metal particle    -   24: Oxide layer    -   25: Surface-coated silicon carbide particle    -   G: (Simulated) exhaust gas    -   100: Exhaust gas purification filter    -   111: Filter base body    -   112: Gas flow passage    -   112A: Inflow cell    -   112B: Outflow cell    -   113: Porous film    -   114: Partition wall    -   121 a, 121 b: Noble-metal-carrying silicon carbide particle    -   122: Silicon carbide particle    -   123: Noble metal particle    -   124: Oxide layer    -   125: Surface-coated silicon carbide particle    -   130: Particulate matter    -   α, γ: End face    -   C: Purified gas

[FIG. 3]

-   -   POROUS FILM    -   BASE MATERIAL

[FIG. 6]

-   -   Pt PARTICLE    -   OXIDE FILM    -   SiC PARTICLE    -   OTHER SiC PARTICLES

[FIG. 10]

-   -   POROUS FILM    -   BASE MATERIAL

[FIG. 12]

-   -   OTHER SiC PARTICLES    -   Pt PARTICLE    -   OXIDE FILM    -   SiC PARTICLE    -   OTHER SiC PARTICLES

1. An exhaust purification catalyst in which a noble metal particle iscarried on a surface of a silicon carbide particle, wherein the noblemetal particle is carried in a state of being coated with an oxidelayer.
 2. The exhaust purification catalyst according to claim 1,wherein the oxide layer is formed from one or two compounds selectedfrom a group consisting of amorphous SiO_(x) (provided that, 0<x≦3) andamorphous SiO_(y)C_(z) (provided that, 0<y≦3 and 0<z≦3).
 3. The exhaustpurification catalyst according to claim 2, wherein the oxide layerfurther contains one or two or more crystalline substances selected froma group consisting of SiO₂, SiO, SiOC₃, SiO₂C₂, and SiO₃C.
 4. Theexhaust purification catalyst according to claim 1, wherein an averageprimary particle size of the silicon carbide particle is 0.01 μm to 5μm.
 5. The exhaust purification catalyst according to claim 1, whereinan average primary particle size of the noble metal particle is 1 nm to50 nm.
 6. An exhaust emission control device for an internal combustionengine, which purifies an exhaust gas discharged from an internalcombustion engine by using a catalyst disposed in an exhaust passage ofthe internal combustion engine, wherein at least one catalyst is theexhaust purification catalyst according to claim
 1. 7. An exhaust gaspurification filter which purifies an exhaust gas by allowingparticulate matter contained in the exhaust gas to pass through a filterbase body formed from a porous substance to trap the particulate matter,wherein the filter base body includes, a partition wall that is formedfrom a porous substance; an inflow-side gas flow passage which is formedby the partition wall and in which an inflow-side end for an exhaust gasthat contains particulate matter is opened; and an outflow-side gas flowpassage which is provided at a position different from that of theinflow-side gas flow passage of the filter base body and is formed bythe partition wall and in which an outflow-side end for the exhaust gasis opened, wherein a porous film having a pore size smaller than that ofthe partition wall is formed on a surface of the partition wall at leaston an inflow-side gas flow passage side, and the porous film containssilicon carbide particles and noble metal fine particles, and the noblemetal particles, which are carried on the surface of the silicon carbideparticles, are carried in a state of being coated with an oxide layer.8. The exhaust gas purification filter according to claim 7, wherein theoxide layer is formed from one or two compounds selected from a groupconsisting of amorphous SiO_(x) (provided that, 0<x≦3) and amorphousSiO_(y)C_(z) (provided that, 0<y≦3 and 0<z≦3).
 9. The exhaust gaspurification filter according to claim 8, wherein the oxide layerfurther contains one or two or more crystalline substances selected froma group consisting of SiO₂, SiO, SiOC₃, SiO₂C₂, and SiO₃C.
 10. Theexhaust gas purification filter according to claim 7, wherein an averageprimary particle size of the silicon carbide particles is 0.01 μm to 10μm.
 11. The exhaust gas purification filter according to claim 7,wherein an average primary particle size of the noble metal particles is1 nm to 50 nm.
 12. The exhaust gas purification filter according toclaim 7, wherein an average pore size of the porous film is 0.05 μm to 3μm, and an average porosity of the porous film is 50% to 90%.
 13. Theexhaust purification catalyst according to claim 2, wherein an averageprimary particle size of the silicon carbide particle is 0.01 μm to 5μm.
 14. The exhaust purification catalyst according to claim 2, whereinan average primary particle size of the noble metal particle is 1 nm to50 nm.
 15. An exhaust emission control device for an internal combustionengine, which purifies an exhaust gas discharged from an internalcombustion engine by using a catalyst disposed in an exhaust passage ofthe internal combustion engine, wherein at least one catalyst is theexhaust purification catalyst according to claim
 2. 16. The exhaust gaspurification filter according to claim 8, wherein an average primaryparticle size of the silicon carbide particles is 0.01 μm to 10 μm. 17.The exhaust gas purification filter according to claim 8, wherein anaverage primary particle size of the noble metal particles is 1 nm to 50nm.
 18. The exhaust gas purification filter according to claim 8,wherein an average pore size of the porous film is 0.05 μm to 3 μm, andan average porosity of the porous film is 50% to 90%.